Settings
distributed_product_mode
Changes the behaviour of distributed subqueries.
ClickHouse applies this setting when the query contains the product of distributed tables, i.e. when the query for a distributed table contains a non-GLOBAL subquery for the distributed table.
Restrictions:
- Only applied for IN and JOIN subqueries.
- Only if the FROM section uses a distributed table containing more than one shard.
- If the subquery concerns a distributed table containing more than one shard.
- Not used for a table-valued remote function.
Possible values:
deny
— Default value. Prohibits using these types of subqueries (returns the “Double-distributed in/JOIN subqueries is denied” exception).local
— Replaces the database and table in the subquery with local ones for the destination server (shard), leaving the normalIN
/JOIN.
global
— Replaces theIN
/JOIN
query withGLOBAL IN
/GLOBAL JOIN.
allow
— Allows the use of these types of subqueries.
prefer_global_in_and_join
Enables the replacement of IN
/JOIN
operators with GLOBAL IN
/GLOBAL JOIN
.
Possible values:
- 0 — Disabled.
IN
/JOIN
operators are not replaced withGLOBAL IN
/GLOBAL JOIN
. - 1 — Enabled.
IN
/JOIN
operators are replaced withGLOBAL IN
/GLOBAL JOIN
.
Default value: 0
.
Usage
Although SET distributed_product_mode=global
can change the queries behavior for the distributed tables, it's not suitable for local tables or tables from external resources. Here is when the prefer_global_in_and_join
setting comes into play.
For example, we have query serving nodes that contain local tables, which are not suitable for distribution. We need to scatter their data on the fly during distributed processing with the GLOBAL
keyword — GLOBAL IN
/GLOBAL JOIN
.
Another use case of prefer_global_in_and_join
is accessing tables created by external engines. This setting helps to reduce the number of calls to external sources while joining such tables: only one call per query.
See also:
- Distributed subqueries for more information on how to use
GLOBAL IN
/GLOBAL JOIN
enable_optimize_predicate_expression
Turns on predicate pushdown in SELECT
queries.
Predicate pushdown may significantly reduce network traffic for distributed queries.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 1.
Usage
Consider the following queries:
SELECT count() FROM test_table WHERE date = '2018-10-10'
SELECT count() FROM (SELECT * FROM test_table) WHERE date = '2018-10-10'
If enable_optimize_predicate_expression = 1
, then the execution time of these queries is equal because ClickHouse applies WHERE
to the subquery when processing it.
If enable_optimize_predicate_expression = 0
, then the execution time of the second query is much longer because the WHERE
clause applies to all the data after the subquery finishes.
fallback_to_stale_replicas_for_distributed_queries
Forces a query to an out-of-date replica if updated data is not available. See Replication.
ClickHouse selects the most relevant from the outdated replicas of the table.
Used when performing SELECT
from a distributed table that points to replicated tables.
By default, 1 (enabled).
force_index_by_date
Disables query execution if the index can’t be used by date.
Works with tables in the MergeTree family.
If force_index_by_date=1
, ClickHouse checks whether the query has a date key condition that can be used for restricting data ranges. If there is no suitable condition, it throws an exception. However, it does not check whether the condition reduces the amount of data to read. For example, the condition Date != ' 2000-01-01 '
is acceptable even when it matches all the data in the table (i.e., running the query requires a full scan). For more information about ranges of data in MergeTree tables, see MergeTree.
force_primary_key
Disables query execution if indexing by the primary key is not possible.
Works with tables in the MergeTree family.
If force_primary_key=1
, ClickHouse checks to see if the query has a primary key condition that can be used for restricting data ranges. If there is no suitable condition, it throws an exception. However, it does not check whether the condition reduces the amount of data to read. For more information about data ranges in MergeTree tables, see MergeTree.
use_skip_indexes
Use data skipping indexes during query execution.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 1.
force_data_skipping_indices
Disables query execution if passed data skipping indices wasn't used.
Consider the following example:
CREATE TABLE data
(
key Int,
d1 Int,
d1_null Nullable(Int),
INDEX d1_idx d1 TYPE minmax GRANULARITY 1,
INDEX d1_null_idx assumeNotNull(d1_null) TYPE minmax GRANULARITY 1
)
Engine=MergeTree()
ORDER BY key;
SELECT * FROM data_01515;
SELECT * FROM data_01515 SETTINGS force_data_skipping_indices=''; -- query will produce CANNOT_PARSE_TEXT error.
SELECT * FROM data_01515 SETTINGS force_data_skipping_indices='d1_idx'; -- query will produce INDEX_NOT_USED error.
SELECT * FROM data_01515 WHERE d1 = 0 SETTINGS force_data_skipping_indices='d1_idx'; -- Ok.
SELECT * FROM data_01515 WHERE d1 = 0 SETTINGS force_data_skipping_indices='`d1_idx`'; -- Ok (example of full featured parser).
SELECT * FROM data_01515 WHERE d1 = 0 SETTINGS force_data_skipping_indices='`d1_idx`, d1_null_idx'; -- query will produce INDEX_NOT_USED error, since d1_null_idx is not used.
SELECT * FROM data_01515 WHERE d1 = 0 AND assumeNotNull(d1_null) = 0 SETTINGS force_data_skipping_indices='`d1_idx`, d1_null_idx'; -- Ok.
Works with tables in the MergeTree family.
format_schema
This parameter is useful when you are using formats that require a schema definition, such as Cap’n Proto or Protobuf. The value depends on the format.
fsync_metadata
Enables or disables fsync when writing .sql
files. Enabled by default.
It makes sense to disable it if the server has millions of tiny tables that are constantly being created and destroyed.
function_range_max_elements_in_block
Sets the safety threshold for data volume generated by function range. Defines the maximum number of values generated by function per block of data (sum of array sizes for every row in a block).
Possible values:
- Positive integer.
Default value: 500,000,000
.
See Also
enable_http_compression
Enables or disables data compression in the response to an HTTP request.
For more information, read the HTTP interface description.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0.
http_zlib_compression_level
Sets the level of data compression in the response to an HTTP request if enable_http_compression = 1.
Possible values: Numbers from 1 to 9.
Default value: 3.
http_native_compression_disable_checksumming_on_decompress
Enables or disables checksum verification when decompressing the HTTP POST data from the client. Used only for ClickHouse native compression format (not used with gzip
or deflate
).
For more information, read the HTTP interface description.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0.
http_max_uri_size
Sets the maximum URI length of an HTTP request.
Possible values:
- Positive integer.
Default value: 1048576.
table_function_remote_max_addresses
Sets the maximum number of addresses generated from patterns for the remote function.
Possible values:
- Positive integer.
Default value: 1000
.
glob_expansion_max_elements
Sets the maximum number of addresses generated from patterns for external storages and table functions (like url) except the remote
function.
Possible values:
- Positive integer.
Default value: 1000
.
send_progress_in_http_headers
Enables or disables X-ClickHouse-Progress
HTTP response headers in clickhouse-server
responses.
For more information, read the HTTP interface description.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0.
max_http_get_redirects
Limits the maximum number of HTTP GET redirect hops for URL-engine tables. The setting applies to both types of tables: those created by the CREATE TABLE query and by the url table function.
Possible values:
- Any positive integer number of hops.
- 0 — No hops allowed.
Default value: 0.
input_format_allow_errors_num
Sets the maximum number of acceptable errors when reading from text formats (CSV, TSV, etc.).
The default value is 0.
Always pair it with input_format_allow_errors_ratio
.
If an error occurred while reading rows but the error counter is still less than input_format_allow_errors_num
, ClickHouse ignores the row and moves on to the next one.
If both input_format_allow_errors_num
and input_format_allow_errors_ratio
are exceeded, ClickHouse throws an exception.
input_format_allow_errors_ratio
Sets the maximum percentage of errors allowed when reading from text formats (CSV, TSV, etc.).
The percentage of errors is set as a floating-point number between 0 and 1.
The default value is 0.
Always pair it with input_format_allow_errors_num
.
If an error occurred while reading rows but the error counter is still less than input_format_allow_errors_ratio
, ClickHouse ignores the row and moves on to the next one.
If both input_format_allow_errors_num
and input_format_allow_errors_ratio
are exceeded, ClickHouse throws an exception.
input_format_parquet_import_nested
Enables or disables the ability to insert the data into Nested columns as an array of structs in Parquet input format.
Possible values:
- 0 — Data can not be inserted into
Nested
columns as an array of structs. - 1 — Data can be inserted into
Nested
columns as an array of structs.
Default value: 0
.
input_format_arrow_import_nested
Enables or disables the ability to insert the data into Nested columns as an array of structs in Arrow input format.
Possible values:
- 0 — Data can not be inserted into
Nested
columns as an array of structs. - 1 — Data can be inserted into
Nested
columns as an array of structs.
Default value: 0
.
input_format_orc_import_nested
Enables or disables the ability to insert the data into Nested columns as an array of structs in ORC input format.
Possible values:
- 0 — Data can not be inserted into
Nested
columns as an array of structs. - 1 — Data can be inserted into
Nested
columns as an array of structs.
Default value: 0
.
input_format_values_interpret_expressions
Enables or disables the full SQL parser if the fast stream parser can’t parse the data. This setting is used only for the Values format at the data insertion. For more information about syntax parsing, see the Syntax section.
Possible values:
-
0 — Disabled.
In this case, you must provide formatted data. See the Formats section.
-
1 — Enabled.
In this case, you can use an SQL expression as a value, but data insertion is much slower this way. If you insert only formatted data, then ClickHouse behaves as if the setting value is 0.
Default value: 1.
Example of Use
Insert the DateTime type value with the different settings.
SET input_format_values_interpret_expressions = 0;
INSERT INTO datetime_t VALUES (now())
Exception on client:
Code: 27. DB::Exception: Cannot parse input: expected ) before: now()): (at row 1)
SET input_format_values_interpret_expressions = 1;
INSERT INTO datetime_t VALUES (now())
Ok.
The last query is equivalent to the following:
SET input_format_values_interpret_expressions = 0;
INSERT INTO datetime_t SELECT now()
Ok.
input_format_values_deduce_templates_of_expressions
Enables or disables template deduction for SQL expressions in Values format. It allows parsing and interpreting expressions in Values
much faster if expressions in consecutive rows have the same structure. ClickHouse tries to deduce the template of an expression, parse the following rows using this template and evaluate the expression on a batch of successfully parsed rows.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 1.
For the following query:
INSERT INTO test VALUES (lower('Hello')), (lower('world')), (lower('INSERT')), (upper('Values')), ...
- If
input_format_values_interpret_expressions=1
andformat_values_deduce_templates_of_expressions=0
, expressions are interpreted separately for each row (this is very slow for large number of rows). - If
input_format_values_interpret_expressions=0
andformat_values_deduce_templates_of_expressions=1
, expressions in the first, second and third rows are parsed using templatelower(String)
and interpreted together, expression in the forth row is parsed with another template (upper(String)
). - If
input_format_values_interpret_expressions=1
andformat_values_deduce_templates_of_expressions=1
, the same as in previous case, but also allows fallback to interpreting expressions separately if it’s not possible to deduce template.
input_format_values_accurate_types_of_literals
This setting is used only when input_format_values_deduce_templates_of_expressions = 1
. Expressions for some column may have the same structure, but contain numeric literals of different types, e.g.
(..., abs(0), ...), -- UInt64 literal
(..., abs(3.141592654), ...), -- Float64 literal
(..., abs(-1), ...), -- Int64 literal
Possible values:
-
0 — Disabled.
In this case, ClickHouse may use a more general type for some literals (e.g.,
Float64
orInt64
instead ofUInt64
for42
), but it may cause overflow and precision issues. -
1 — Enabled.
In this case, ClickHouse checks the actual type of literal and uses an expression template of the corresponding type. In some cases, it may significantly slow down expression evaluation in
Values
.
Default value: 1.
input_format_defaults_for_omitted_fields
When performing INSERT
queries, replace omitted input column values with default values of the respective columns. This option only applies to JSONEachRow, CSV and TabSeparated formats.
Note
When this option is enabled, extended table metadata are sent from server to client. It consumes additional computing resources on the server and can reduce performance.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 1.
input_format_tsv_empty_as_default
When enabled, replace empty input fields in TSV with default values. For complex default expressions input_format_defaults_for_omitted_fields
must be enabled too.
Disabled by default.
input_format_tsv_enum_as_number
Enables or disables parsing enum values as enum ids for TSV input format.
Possible values:
- 0 — Enum values are parsed as values.
- 1 — Enum values are parsed as enum IDs.
Default value: 0.
Example
Consider the table:
CREATE TABLE table_with_enum_column_for_tsv_insert (Id Int32,Value Enum('first' = 1, 'second' = 2)) ENGINE=Memory();
When the input_format_tsv_enum_as_number
setting is enabled:
SET input_format_tsv_enum_as_number = 1;
INSERT INTO table_with_enum_column_for_tsv_insert FORMAT TSV 102 2;
INSERT INTO table_with_enum_column_for_tsv_insert FORMAT TSV 103 1;
SELECT * FROM table_with_enum_column_for_tsv_insert;
Result:
┌──Id─┬─Value──┐
│ 102 │ second │
└─────┴────────┘
┌──Id─┬─Value──┐
│ 103 │ first │
└─────┴────────┘
When the input_format_tsv_enum_as_number
setting is disabled, the INSERT
query:
SET input_format_tsv_enum_as_number = 0;
INSERT INTO table_with_enum_column_for_tsv_insert FORMAT TSV 102 2;
throws an exception.
input_format_null_as_default
Enables or disables the initialization of NULL fields with default values, if data type of these fields is not nullable.
If column type is not nullable and this setting is disabled, then inserting NULL
causes an exception. If column type is nullable, then NULL
values are inserted as is, regardless of this setting.
This setting is applicable to INSERT ... VALUES queries for text input formats.
Possible values:
- 0 — Inserting
NULL
into a not nullable column causes an exception. - 1 —
NULL
fields are initialized with default column values.
Default value: 1
.
insert_null_as_default
Enables or disables the insertion of default values instead of NULL into columns with not nullable data type.
If column type is not nullable and this setting is disabled, then inserting NULL
causes an exception. If column type is nullable, then NULL
values are inserted as is, regardless of this setting.
This setting is applicable to INSERT ... SELECT queries. Note that SELECT
subqueries may be concatenated with UNION ALL
clause.
Possible values:
- 0 — Inserting
NULL
into a not nullable column causes an exception. - 1 — Default column value is inserted instead of
NULL
.
Default value: 1
.
input_format_skip_unknown_fields
Enables or disables skipping insertion of extra data.
When writing data, ClickHouse throws an exception if input data contain columns that do not exist in the target table. If skipping is enabled, ClickHouse does not insert extra data and does not throw an exception.
Supported formats:
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0.
input_format_import_nested_json
Enables or disables the insertion of JSON data with nested objects.
Supported formats:
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0.
See also:
- Usage of Nested Structures with the
JSONEachRow
format.
input_format_with_names_use_header
Enables or disables checking the column order when inserting data.
To improve insert performance, we recommend disabling this check if you are sure that the column order of the input data is the same as in the target table.
Supported formats:
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 1.
date_time_input_format
Allows choosing a parser of the text representation of date and time.
The setting does not apply to date and time functions.
Possible values:
-
'best_effort'
— Enables extended parsing.ClickHouse can parse the basic
YYYY-MM-DD HH:MM:SS
format and all ISO 8601 date and time formats. For example,'2018-06-08T01:02:03.000Z'
. -
'basic'
— Use basic parser.ClickHouse can parse only the basic
YYYY-MM-DD HH:MM:SS
orYYYY-MM-DD
format. For example,2019-08-20 10:18:56
or2019-08-20
.
Default value: 'basic'
.
See also:
date_time_output_format
Allows choosing different output formats of the text representation of date and time.
Possible values:
-
simple
- Simple output format.ClickHouse output date and time
YYYY-MM-DD hh:mm:ss
format. For example,2019-08-20 10:18:56
. The calculation is performed according to the data type's time zone (if present) or server time zone. -
iso
- ISO output format.ClickHouse output date and time in ISO 8601
YYYY-MM-DDThh:mm:ssZ
format. For example,2019-08-20T10:18:56Z
. Note that output is in UTC (Z
means UTC). -
unix_timestamp
- Unix timestamp output format.ClickHouse output date and time in Unix timestamp format. For example
1566285536
.
Default value: simple
.
See also:
join_default_strictness
Sets default strictness for JOIN clauses.
Possible values:
ALL
— If the right table has several matching rows, ClickHouse creates a Cartesian product from matching rows. This is the normalJOIN
behaviour from standard SQL.ANY
— If the right table has several matching rows, only the first one found is joined. If the right table has only one matching row, the results ofANY
andALL
are the same.ASOF
— For joining sequences with an uncertain match.Empty string
— IfALL
orANY
is not specified in the query, ClickHouse throws an exception.
Default value: ALL
.
join_algorithm
Specifies JOIN algorithm.
Possible values:
hash
— Hash join algorithm is used.partial_merge
— Sort-merge algorithm is used.prefer_partial_merge
— ClickHouse always tries to usemerge
join if possible.auto
— ClickHouse tries to changehash
join tomerge
join on the fly to avoid out of memory.
Default value: hash
.
When using hash
algorithm the right part of JOIN
is uploaded into RAM.
When using partial_merge
algorithm ClickHouse sorts the data and dumps it to the disk. The merge
algorithm in ClickHouse differs a bit from the classic realization. First ClickHouse sorts the right table by join key in blocks and creates min-max index for sorted blocks. Then it sorts parts of left table by join key
and joins them over right table. The min-max index is also used to skip unneeded right table blocks.
join_any_take_last_row
Changes behaviour of join operations with ANY
strictness.
Attention
This setting applies only for JOIN
operations with Join engine tables.
Possible values:
- 0 — If the right table has more than one matching row, only the first one found is joined.
- 1 — If the right table has more than one matching row, only the last one found is joined.
Default value: 0.
See also:
join_use_nulls
Sets the type of JOIN behaviour. When merging tables, empty cells may appear. ClickHouse fills them differently based on this setting.
Possible values:
- 0 — The empty cells are filled with the default value of the corresponding field type.
- 1 —
JOIN
behaves the same way as in standard SQL. The type of the corresponding field is converted to Nullable, and empty cells are filled with NULL.
Default value: 0.
partial_merge_join_optimizations
Disables optimizations in partial merge join algorithm for JOIN queries.
By default, this setting enables improvements that could lead to wrong results. If you see suspicious results in your queries, disable optimizations by this setting. Optimizations can be different in different versions of the ClickHouse server.
Possible values:
- 0 — Optimizations disabled.
- 1 — Optimizations enabled.
Default value: 1.
partial_merge_join_rows_in_right_blocks
Limits sizes of right-hand join data blocks in partial merge join algorithm for JOIN queries.
ClickHouse server:
- Splits right-hand join data into blocks with up to the specified number of rows.
- Indexes each block with its minimum and maximum values.
- Unloads prepared blocks to disk if it is possible.
Possible values:
- Any positive integer. Recommended range of values: [1000, 100000].
Default value: 65536.
join_on_disk_max_files_to_merge
Limits the number of files allowed for parallel sorting in MergeJoin operations when they are executed on disk.
The bigger the value of the setting, the more RAM used and the less disk I/O needed.
Possible values:
- Any positive integer, starting from 2.
Default value: 64.
any_join_distinct_right_table_keys
Enables legacy ClickHouse server behaviour in ANY INNER|LEFT JOIN
operations.
Warning
Use this setting only for backward compatibility if your use cases depend on legacy JOIN
behaviour.
When the legacy behaviour enabled:
- Results of
t1 ANY LEFT JOIN t2
andt2 ANY RIGHT JOIN t1
operations are not equal because ClickHouse uses the logic with many-to-one left-to-right table keys mapping. - Results of
ANY INNER JOIN
operations contain all rows from the left table like theSEMI LEFT JOIN
operations do.
When the legacy behaviour disabled:
- Results of
t1 ANY LEFT JOIN t2
andt2 ANY RIGHT JOIN t1
operations are equal because ClickHouse uses the logic which provides one-to-many keys mapping inANY RIGHT JOIN
operations. - Results of
ANY INNER JOIN
operations contain one row per key from both the left and right tables.
Possible values:
- 0 — Legacy behaviour is disabled.
- 1 — Legacy behaviour is enabled.
Default value: 0.
See also:
temporary_files_codec
Sets compression codec for temporary files used in sorting and joining operations on disk.
Possible values:
- LZ4 — LZ4 compression is applied.
- NONE — No compression is applied.
Default value: LZ4.
max_block_size
In ClickHouse, data is processed by blocks (sets of column parts). The internal processing cycles for a single block are efficient enough, but there are noticeable expenditures on each block. The max_block_size
setting is a recommendation for what size of the block (in a count of rows) to load from tables. The block size shouldn’t be too small, so that the expenditures on each block are still noticeable, but not too large so that the query with LIMIT that is completed after the first block is processed quickly. The goal is to avoid consuming too much memory when extracting a large number of columns in multiple threads and to preserve at least some cache locality.
Default value: 65,536.
Blocks the size of max_block_size
are not always loaded from the table. If it is obvious that less data needs to be retrieved, a smaller block is processed.
preferred_block_size_bytes
Used for the same purpose as max_block_size
, but it sets the recommended block size in bytes by adapting it to the number of rows in the block.
However, the block size cannot be more than max_block_size
rows.
By default: 1,000,000. It only works when reading from MergeTree engines.
merge_tree_min_rows_for_concurrent_read
If the number of rows to be read from a file of a MergeTree table exceeds merge_tree_min_rows_for_concurrent_read
then ClickHouse tries to perform a concurrent reading from this file on several threads.
Possible values:
- Any positive integer.
Default value: 163840.
merge_tree_min_bytes_for_concurrent_read
If the number of bytes to read from one file of a MergeTree-engine table exceeds merge_tree_min_bytes_for_concurrent_read
, then ClickHouse tries to concurrently read from this file in several threads.
Possible value:
- Any positive integer.
Default value: 251658240.
merge_tree_min_rows_for_seek
If the distance between two data blocks to be read in one file is less than merge_tree_min_rows_for_seek
rows, then ClickHouse does not seek through the file but reads the data sequentially.
Possible values:
- Any positive integer.
Default value: 0.
merge_tree_min_bytes_for_seek
If the distance between two data blocks to be read in one file is less than merge_tree_min_bytes_for_seek
bytes, then ClickHouse sequentially reads a range of file that contains both blocks, thus avoiding extra seek.
Possible values:
- Any positive integer.
Default value: 0.
merge_tree_coarse_index_granularity
When searching for data, ClickHouse checks the data marks in the index file. If ClickHouse finds that required keys are in some range, it divides this range into merge_tree_coarse_index_granularity
subranges and searches the required keys there recursively.
Possible values:
- Any positive even integer.
Default value: 8.
merge_tree_max_rows_to_use_cache
If ClickHouse should read more than merge_tree_max_rows_to_use_cache
rows in one query, it does not use the cache of uncompressed blocks.
The cache of uncompressed blocks stores data extracted for queries. ClickHouse uses this cache to speed up responses to repeated small queries. This setting protects the cache from trashing by queries that read a large amount of data. The uncompressed_cache_size server setting defines the size of the cache of uncompressed blocks.
Possible values:
- Any positive integer.
Default value: 128 ✕ 8192.
merge_tree_max_bytes_to_use_cache
If ClickHouse should read more than merge_tree_max_bytes_to_use_cache
bytes in one query, it does not use the cache of uncompressed blocks.
The cache of uncompressed blocks stores data extracted for queries. ClickHouse uses this cache to speed up responses to repeated small queries. This setting protects the cache from trashing by queries that read a large amount of data. The uncompressed_cache_size server setting defines the size of the cache of uncompressed blocks.
Possible values:
- Any positive integer.
Default value:<PHONE_NUMBER>.
merge_tree_clear_old_temporary_directories_interval_seconds
Sets the interval in seconds for ClickHouse to execute the cleanup of old temporary directories.
Possible values:
- Any positive integer.
Default value: 60
seconds.
merge_tree_clear_old_parts_interval_seconds
Sets the interval in seconds for ClickHouse to execute the cleanup of old parts, WALs, and mutations.
Possible values:
- Any positive integer.
Default value: 1
second.
min_bytes_to_use_direct_io
The minimum data volume required for using direct I/O access to the storage disk.
ClickHouse uses this setting when reading data from tables. If the total storage volume of all the data to be read exceeds min_bytes_to_use_direct_io
bytes, then ClickHouse reads the data from the storage disk with the O_DIRECT
option.
Possible values:
- 0 — Direct I/O is disabled.
- Positive integer.
Default value: 0.
network_compression_method
Sets the method of data compression that is used for communication between servers and between server and clickhouse-client.
Possible values:
LZ4
— sets LZ4 compression method.ZSTD
— sets ZSTD compression method.
Default value: LZ4
.
See Also
network_zstd_compression_level
Adjusts the level of ZSTD compression. Used only when network_compression_method is set to ZSTD
.
Possible values:
- Positive integer from 1 to 15.
Default value: 1
.
log_queries
Setting up query logging.
Queries sent to ClickHouse with this setup are logged according to the rules in the query_log server configuration parameter.
Example:
log_queries=1
log_queries_min_query_duration_ms
If enabled (non-zero), queries faster then the value of this setting will not be logged (you can think about this as a long_query_time
for MySQL Slow Query Log), and this basically means that you will not find them in the following tables:
system.query_log
system.query_thread_log
Only the queries with the following type will get to the log:
QUERY_FINISH
-
EXCEPTION_WHILE_PROCESSING
-
Type: milliseconds
- Default value: 0 (any query)
log_queries_min_type
query_log
minimal type to log.
Possible values:
- QUERY_START
(=1
)
- QUERY_FINISH
(=2
)
- EXCEPTION_BEFORE_START
(=3
)
- EXCEPTION_WHILE_PROCESSING
(=4
)
Default value: QUERY_START
.
Can be used to limit which entities will go to query_log
, say you are interested only in errors, then you can use EXCEPTION_WHILE_PROCESSING
:
log_queries_min_type='EXCEPTION_WHILE_PROCESSING'
log_query_threads
Setting up query threads logging.
Queries’ threads run by ClickHouse with this setup are logged according to the rules in the query_thread_log server configuration parameter.
Example:
log_query_threads=1
log_query_views
Setting up query views logging.
When a query run by ClickHouse with this setup on has associated views (materialized or live views), they are logged in the query_views_log server configuration parameter.
Example:
log_query_views=1
log_comment
Specifies the value for the log_comment
field of the system.query_log table and comment text for the server log.
It can be used to improve the readability of server logs. Additionally, it helps to select queries related to the test from the system.query_log
after running clickhouse-test.
Possible values:
- Any string no longer than max_query_size. If length is exceeded, the server throws an exception.
Default value: empty string.
Example
Query:
SET log_comment = 'log_comment test', log_queries = 1;
SELECT 1;
SYSTEM FLUSH LOGS;
SELECT type, query FROM system.query_log WHERE log_comment = 'log_comment test' AND event_date >= yesterday() ORDER BY event_time DESC LIMIT 2;
Result:
┌─type────────┬─query─────┐
│ QueryStart │ SELECT 1; │
│ QueryFinish │ SELECT 1; │
└─────────────┴───────────┘
max_insert_block_size
The size of blocks (in a count of rows) to form for insertion into a table.
This setting only applies in cases when the server forms the blocks.
For example, for an INSERT via the HTTP interface, the server parses the data format and forms blocks of the specified size.
But when using clickhouse-client, the client parses the data itself, and the ‘max_insert_block_size’ setting on the server does not affect the size of the inserted blocks.
The setting also does not have a purpose when using INSERT SELECT, since data is inserted using the same blocks that are formed after SELECT.
Default value: 1,048,576.
The default is slightly more than max_block_size
. The reason for this is because certain table engines (*MergeTree
) form a data part on the disk for each inserted block, which is a fairly large entity. Similarly, *MergeTree
tables sort data during insertion, and a large enough block size allow sorting more data in RAM.
min_insert_block_size_rows
Sets the minimum number of rows in the block which can be inserted into a table by an INSERT
query. Smaller-sized blocks are squashed into bigger ones.
Possible values:
- Positive integer.
- 0 — Squashing disabled.
Default value: 1048576.
min_insert_block_size_bytes
Sets the minimum number of bytes in the block which can be inserted into a table by an INSERT
query. Smaller-sized blocks are squashed into bigger ones.
Possible values:
- Positive integer.
- 0 — Squashing disabled.
Default value: 268435456.
max_replica_delay_for_distributed_queries
Disables lagging replicas for distributed queries. See Replication.
Sets the time in seconds. If a replica lags more than the set value, this replica is not used.
Default value: 300.
Used when performing SELECT
from a distributed table that points to replicated tables.
max_threads
The maximum number of query processing threads, excluding threads for retrieving data from remote servers (see the ‘max_distributed_connections’ parameter).
This parameter applies to threads that perform the same stages of the query processing pipeline in parallel.
For example, when reading from a table, if it is possible to evaluate expressions with functions, filter with WHERE and pre-aggregate for GROUP BY in parallel using at least ‘max_threads’ number of threads, then ‘max_threads’ are used.
Default value: the number of physical CPU cores.
For queries that are completed quickly because of a LIMIT, you can set a lower ‘max_threads’. For example, if the necessary number of entries are located in every block and max_threads = 8, then 8 blocks are retrieved, although it would have been enough to read just one.
The smaller the max_threads
value, the less memory is consumed.
max_insert_threads
The maximum number of threads to execute the INSERT SELECT
query.
Possible values:
- 0 (or 1) —
INSERT SELECT
no parallel execution. - Positive integer. Bigger than 1.
Default value: 0.
Parallel INSERT SELECT
has effect only if the SELECT
part is executed in parallel, see max_threads setting.
Higher values will lead to higher memory usage.
max_compress_block_size
The maximum size of blocks of uncompressed data before compressing for writing to a table. By default, 1,048,576 (1 MiB). Specifying smaller block size generally leads to slightly reduced compression ratio, the compression and decompression speed increases slightly due to cache locality, and memory consumption is reduced.
Warning
This is an expert-level setting, and you shouldn't change it if you're just getting started with ClickHouse.
Don’t confuse blocks for compression (a chunk of memory consisting of bytes) with blocks for query processing (a set of rows from a table).
min_compress_block_size
For MergeTree tables. In order to reduce latency when processing queries, a block is compressed when writing the next mark if its size is at least min_compress_block_size
. By default, 65,536.
The actual size of the block, if the uncompressed data is less than max_compress_block_size
, is no less than this value and no less than the volume of data for one mark.
Let’s look at an example. Assume that index_granularity
was set to 8192 during table creation.
We are writing a UInt32-type column (4 bytes per value). When writing 8192 rows, the total will be 32 KB of data. Since min_compress_block_size = 65,536, a compressed block will be formed for every two marks.
We are writing a URL column with the String type (average size of 60 bytes per value). When writing 8192 rows, the average will be slightly less than 500 KB of data. Since this is more than 65,536, a compressed block will be formed for each mark. In this case, when reading data from the disk in the range of a single mark, extra data won’t be decompressed.
Warning
This is an expert-level setting, and you shouldn't change it if you're just getting started with ClickHouse.
max_query_size
The maximum part of a query that can be taken to RAM for parsing with the SQL parser.
The INSERT query also contains data for INSERT that is processed by a separate stream parser (that consumes O(1) RAM), which is not included in this restriction.
Default value: 256 KiB.
max_parser_depth
Limits maximum recursion depth in the recursive descent parser. Allows controlling the stack size.
Possible values:
- Positive integer.
- 0 — Recursion depth is unlimited.
Default value: 1000.
interactive_delay
The interval in microseconds for checking whether request execution has been cancelled and sending the progress.
Default value: 100,000 (checks for cancelling and sends the progress ten times per second).
connect_timeout, receive_timeout, send_timeout
Timeouts in seconds on the socket used for communicating with the client.
Default value: 10, 300, 300.
cancel_http_readonly_queries_on_client_close
Cancels HTTP read-only queries (e.g. SELECT) when a client closes the connection without waiting for the response.
Default value: 0
poll_interval
Lock in a wait loop for the specified number of seconds.
Default value: 10.
max_distributed_connections
The maximum number of simultaneous connections with remote servers for distributed processing of a single query to a single Distributed table. We recommend setting a value no less than the number of servers in the cluster.
Default value: 1024.
The following parameters are only used when creating Distributed tables (and when launching a server), so there is no reason to change them at runtime.
distributed_connections_pool_size
The maximum number of simultaneous connections with remote servers for distributed processing of all queries to a single Distributed table. We recommend setting a value no less than the number of servers in the cluster.
Default value: 1024.
max_distributed_depth
Limits the maximum depth of recursive queries for Distributed tables.
If the value is exceeded, the server throws an exception.
Possible values:
- Positive integer.
- 0 — Unlimited depth.
Default value: 5
.
max_replicated_fetches_network_bandwidth_for_server
Limits the maximum speed of data exchange over the network in bytes per second for replicated fetches for the server. Only has meaning at server startup. You can also limit the speed for a particular table with max_replicated_fetches_network_bandwidth setting.
The setting isn't followed perfectly accurately.
Possible values:
- Positive integer.
- 0 — Unlimited.
Default value: 0
.
Usage
Could be used for throttling speed when replicating the data to add or replace new nodes.
max_replicated_sends_network_bandwidth_for_server
Limits the maximum speed of data exchange over the network in bytes per second for replicated sends for the server. Only has meaning at server startup. You can also limit the speed for a particular table with max_replicated_sends_network_bandwidth setting.
The setting isn't followed perfectly accurately.
Possible values:
- Positive integer.
- 0 — Unlimited.
Default value: 0
.
Usage
Could be used for throttling speed when replicating the data to add or replace new nodes.
connect_timeout_with_failover_ms
The timeout in milliseconds for connecting to a remote server for a Distributed table engine, if the ‘shard’ and ‘replica’ sections are used in the cluster definition.
If unsuccessful, several attempts are made to connect to various replicas.
Default value: 50.
connection_pool_max_wait_ms
The wait time in milliseconds for a connection when the connection pool is full.
Possible values:
- Positive integer.
- 0 — Infinite timeout.
Default value: 0.
connections_with_failover_max_tries
The maximum number of connection attempts with each replica for the Distributed table engine.
Default value: 3.
extremes
Whether to count extreme values (the minimums and maximums in columns of a query result). Accepts 0 or 1. By default, 0 (disabled).
For more information, see the section “Extreme values”.
kafka_max_wait_ms
The wait time in milliseconds for reading messages from Kafka before retry.
Possible values:
- Positive integer.
- 0 — Infinite timeout.
Default value: 5000.
See also:
use_uncompressed_cache
Whether to use a cache of uncompressed blocks. Accepts 0 or 1. By default, 0 (disabled).
Using the uncompressed cache (only for tables in the MergeTree family) can significantly reduce latency and increase throughput when working with a large number of short queries. Enable this setting for users who send frequent short requests. Also pay attention to the uncompressed_cache_size configuration parameter (only set in the config file) – the size of uncompressed cache blocks. By default, it is 8 GiB. The uncompressed cache is filled in as needed and the least-used data is automatically deleted.
For queries that read at least a somewhat large volume of data (one million rows or more), the uncompressed cache is disabled automatically to save space for truly small queries. This means that you can keep the ‘use_uncompressed_cache’ setting always set to 1.
replace_running_query
When using the HTTP interface, the ‘query_id’ parameter can be passed. This is any string that serves as the query identifier.
If a query from the same user with the same ‘query_id’ already exists at this time, the behaviour depends on the ‘replace_running_query’ parameter.
0
(default) – Throw an exception (do not allow the query to run if a query with the same ‘query_id’ is already running).
1
– Cancel the old query and start running the new one.
Yandex.Metrica uses this parameter set to 1 for implementing suggestions for segmentation conditions. After entering the next character, if the old query hasn’t finished yet, it should be cancelled.
replace_running_query_max_wait_ms
The wait time for running the query with the same query_id
to finish, when the replace_running_query setting is active.
Possible values:
- Positive integer.
- 0 — Throwing an exception that does not allow to run a new query if the server already executes a query with the same
query_id
.
Default value: 5000.
stream_flush_interval_ms
Works for tables with streaming in the case of a timeout, or when a thread generates max_insert_block_size rows.
The default value is 7500.
The smaller the value, the more often data is flushed into the table. Setting the value too low leads to poor performance.
load_balancing
Specifies the algorithm of replicas selection that is used for distributed query processing.
ClickHouse supports the following algorithms of choosing replicas:
- Random (by default)
- Nearest hostname
- In order
- First or random
- Round robin
See also:
Random (by Default)
load_balancing = random
The number of errors is counted for each replica. The query is sent to the replica with the fewest errors, and if there are several of these, to anyone of them.
Disadvantages: Server proximity is not accounted for; if the replicas have different data, you will also get different data.
Nearest Hostname
load_balancing = nearest_hostname
The number of errors is counted for each replica. Every 5 minutes, the number of errors is integrally divided by 2. Thus, the number of errors is calculated for a recent time with exponential smoothing. If there is one replica with a minimal number of errors (i.e. errors occurred recently on the other replicas), the query is sent to it. If there are multiple replicas with the same minimal number of errors, the query is sent to the replica with a hostname that is most similar to the server’s hostname in the config file (for the number of different characters in identical positions, up to the minimum length of both hostnames).
For instance, example01-01-1 and example01-01-2.yandex.ru are different in one position, while example01-01-1 and example01-02-2 differ in two places.
This method might seem primitive, but it does not require external data about network topology, and it does not compare IP addresses, which would be complicated for our IPv6 addresses.
Thus, if there are equivalent replicas, the closest one by name is preferred.
We can also assume that when sending a query to the same server, in the absence of failures, a distributed query will also go to the same servers. So even if different data is placed on the replicas, the query will return mostly the same results.
In Order
load_balancing = in_order
Replicas with the same number of errors are accessed in the same order as they are specified in the configuration.
This method is appropriate when you know exactly which replica is preferable.
First or Random
load_balancing = first_or_random
This algorithm chooses the first replica in the set or a random replica if the first is unavailable. It’s effective in cross-replication topology setups, but useless in other configurations.
The first_or_random
algorithm solves the problem of the in_order
algorithm. With in_order
, if one replica goes down, the next one gets a double load while the remaining replicas handle the usual amount of traffic. When using the first_or_random
algorithm, the load is evenly distributed among replicas that are still available.
It's possible to explicitly define what the first replica is by using the setting load_balancing_first_offset
. This gives more control to rebalance query workloads among replicas.
Round Robin
load_balancing = round_robin
This algorithm uses a round-robin policy across replicas with the same number of errors (only the queries with round_robin
policy is accounted).
prefer_localhost_replica
Enables/disables preferable using the localhost replica when processing distributed queries.
Possible values:
- 1 — ClickHouse always sends a query to the localhost replica if it exists.
- 0 — ClickHouse uses the balancing strategy specified by the load_balancing setting.
Default value: 1.
Warning
Disable this setting if you use max_parallel_replicas.
totals_mode
How to calculate TOTALS when HAVING is present, as well as when max_rows_to_group_by and group_by_overflow_mode = ‘any’ are present.
See the section “WITH TOTALS modifier”.
totals_auto_threshold
The threshold for totals_mode = 'auto'
.
See the section “WITH TOTALS modifier”.
max_parallel_replicas
The maximum number of replicas for each shard when executing a query.
Possible values:
- Positive integer.
Default value: 1
.
Additional Info
This setting is useful for replicated tables with a sampling key. A query may be processed faster if it is executed on several servers in parallel. But the query performance may degrade in the following cases:
- The position of the sampling key in the partitioning key does not allow efficient range scans.
- Adding a sampling key to the table makes filtering by other columns less efficient.
- The sampling key is an expression that is expensive to calculate.
- The cluster latency distribution has a long tail, so that querying more servers increases the query overall latency.
Warning
This setting will produce incorrect results when joins or subqueries are involved, and all tables don't meet certain requirements. See Distributed Subqueries and max_parallel_replicas for more details.
compile_expressions
Enables or disables compilation of frequently used simple functions and operators to native code with LLVM at runtime.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 1
.
min_count_to_compile_expression
Minimum count of executing same expression before it is get compiled.
Default value: 3
.
output_format_json_quote_64bit_integers
Controls quoting of 64-bit or bigger integers (like UInt64
or Int128
) when they are output in a JSON format.
Such integers are enclosed in quotes by default. This behavior is compatible with most JavaScript implementations.
Possible values:
- 0 — Integers are output without quotes.
- 1 — Integers are enclosed in quotes.
Default value: 1.
output_format_json_quote_denormals
Enables +nan
, -nan
, +inf
, -inf
outputs in JSON output format.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0.
Example
Consider the following table account_orders
:
┌─id─┬─name───┬─duration─┬─period─┬─area─┐
│ 1 │ Andrew │ 20 │ 0 │ 400 │
│ 2 │ John │ 40 │ 0 │ 0 │
│ 3 │ Bob │ 15 │ 0 │ -100 │
└────┴────────┴──────────┴────────┴──────┘
When output_format_json_quote_denormals = 0
, the query returns null
values in output:
SELECT area/period FROM account_orders FORMAT JSON;
{
"meta":
[
{
"name": "divide(area, period)",
"type": "Float64"
}
],
"data":
[
{
"divide(area, period)": null
},
{
"divide(area, period)": null
},
{
"divide(area, period)": null
}
],
"rows": 3,
"statistics":
{
"elapsed": 0.003648093,
"rows_read": 3,
"bytes_read": 24
}
}
When output_format_json_quote_denormals = 1
, the query returns:
{
"meta":
[
{
"name": "divide(area, period)",
"type": "Float64"
}
],
"data":
[
{
"divide(area, period)": "inf"
},
{
"divide(area, period)": "-nan"
},
{
"divide(area, period)": "-inf"
}
],
"rows": 3,
"statistics":
{
"elapsed": 0.000070241,
"rows_read": 3,
"bytes_read": 24
}
}
format_csv_delimiter
The character is interpreted as a delimiter in the CSV data. By default, the delimiter is ,
.
input_format_csv_unquoted_null_literal_as_null
For CSV input format enables or disables parsing of unquoted NULL
as literal (synonym for \N
).
input_format_csv_enum_as_number
Enables or disables parsing enum values as enum ids for CSV input format.
Possible values:
- 0 — Enum values are parsed as values.
- 1 — Enum values are parsed as enum IDs.
Default value: 0.
Examples
Consider the table:
CREATE TABLE table_with_enum_column_for_csv_insert (Id Int32,Value Enum('first' = 1, 'second' = 2)) ENGINE=Memory();
When the input_format_csv_enum_as_number
setting is enabled:
SET input_format_csv_enum_as_number = 1;
INSERT INTO table_with_enum_column_for_csv_insert FORMAT CSV 102,2;
SELECT * FROM table_with_enum_column_for_csv_insert;
Result:
┌──Id─┬─Value─────┐
│ 102 │ second │
└─────┴───────────┘
When the input_format_csv_enum_as_number
setting is disabled, the INSERT
query:
SET input_format_csv_enum_as_number = 0;
INSERT INTO table_with_enum_column_for_csv_insert FORMAT CSV 102,2;
throws an exception.
output_format_csv_crlf_end_of_line
Use DOS/Windows-style line separator (CRLF) in CSV instead of Unix style (LF).
output_format_tsv_crlf_end_of_line
Use DOC/Windows-style line separator (CRLF) in TSV instead of Unix style (LF).
insert_quorum
Enables the quorum writes.
- If
insert_quorum < 2
, the quorum writes are disabled. - If
insert_quorum >= 2
, the quorum writes are enabled.
Default value: 0.
Quorum writes
INSERT
succeeds only when ClickHouse manages to correctly write data to the insert_quorum
of replicas during the insert_quorum_timeout
. If for any reason the number of replicas with successful writes does not reach the insert_quorum
, the write is considered failed and ClickHouse will delete the inserted block from all the replicas where data has already been written.
All the replicas in the quorum are consistent, i.e., they contain data from all previous INSERT
queries. The INSERT
sequence is linearized.
When reading the data written from the insert_quorum
, you can use the select_sequential_consistency option.
ClickHouse generates an exception
- If the number of available replicas at the time of the query is less than the
insert_quorum
. - At an attempt to write data when the previous block has not yet been inserted in the
insert_quorum
of replicas. This situation may occur if the user tries to perform anINSERT
before the previous one with theinsert_quorum
is completed.
See also:
insert_quorum_timeout
Write to a quorum timeout in milliseconds. If the timeout has passed and no write has taken place yet, ClickHouse will generate an exception and the client must repeat the query to write the same block to the same or any other replica.
Default value: 600 000 milliseconds (ten minutes).
See also:
select_sequential_consistency
Enables or disables sequential consistency for SELECT
queries:
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0.
Usage
When sequential consistency is enabled, ClickHouse allows the client to execute the SELECT
query only for those replicas that contain data from all previous INSERT
queries executed with insert_quorum
. If the client refers to a partial replica, ClickHouse will generate an exception. The SELECT query will not include data that has not yet been written to the quorum of replicas.
See also:
insert_deduplicate
Enables or disables block deduplication of INSERT
(for Replicated* tables).
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 1.
By default, blocks inserted into replicated tables by the INSERT
statement are deduplicated (see Data Replication).
deduplicate_blocks_in_dependent_materialized_views
Enables or disables the deduplication check for materialized views that receive data from Replicated* tables.
Possible values:
0 — Disabled.
1 — Enabled.
Default value: 0.
Usage
By default, deduplication is not performed for materialized views but is done upstream, in the source table.
If an INSERTed block is skipped due to deduplication in the source table, there will be no insertion into attached materialized views. This behaviour exists to enable the insertion of highly aggregated data into materialized views, for cases where inserted blocks are the same after materialized view aggregation but derived from different INSERTs into the source table.
At the same time, this behaviour “breaks” INSERT
idempotency. If an INSERT
into the main table was successful and INSERT
into a materialized view failed (e.g. because of communication failure with Zookeeper) a client will get an error and can retry the operation. However, the materialized view won’t receive the second insert because it will be discarded by deduplication in the main (source) table. The setting deduplicate_blocks_in_dependent_materialized_views
allows for changing this behaviour. On retry, a materialized view will receive the repeat insert and will perform a deduplication check by itself,
ignoring check result for the source table, and will insert rows lost because of the first failure.
max_network_bytes
Limits the data volume (in bytes) that is received or transmitted over the network when executing a query. This setting applies to every individual query.
Possible values:
- Positive integer.
- 0 — Data volume control is disabled.
Default value: 0.
max_network_bandwidth
Limits the speed of the data exchange over the network in bytes per second. This setting applies to every query.
Possible values:
- Positive integer.
- 0 — Bandwidth control is disabled.
Default value: 0.
max_network_bandwidth_for_user
Limits the speed of the data exchange over the network in bytes per second. This setting applies to all concurrently running queries performed by a single user.
Possible values:
- Positive integer.
- 0 — Control of the data speed is disabled.
Default value: 0.
max_network_bandwidth_for_all_users
Limits the speed that data is exchanged at over the network in bytes per second. This setting applies to all concurrently running queries on the server.
Possible values:
- Positive integer.
- 0 — Control of the data speed is disabled.
Default value: 0.
count_distinct_implementation
Specifies which of the uniq*
functions should be used to perform the COUNT(DISTINCT …) construction.
Possible values:
Default value: uniqExact
.
skip_unavailable_shards
Enables or disables silently skipping of unavailable shards.
Shard is considered unavailable if all its replicas are unavailable. A replica is unavailable in the following cases:
-
ClickHouse can’t connect to replica for any reason.
When connecting to a replica, ClickHouse performs several attempts. If all these attempts fail, the replica is considered unavailable.
-
Replica can’t be resolved through DNS.
If replica’s hostname can’t be resolved through DNS, it can indicate the following situations:
-
Replica’s host has no DNS record. It can occur in systems with dynamic DNS, for example, Kubernetes, where nodes can be unresolvable during downtime, and this is not an error.
-
Configuration error. ClickHouse configuration file contains a wrong hostname.
-
Possible values:
-
1 — skipping enabled.
If a shard is unavailable, ClickHouse returns a result based on partial data and does not report node availability issues.
-
0 — skipping disabled.
If a shard is unavailable, ClickHouse throws an exception.
Default value: 0.
distributed_group_by_no_merge
Do not merge aggregation states from different servers for distributed query processing, you can use this in case it is for certain that there are different keys on different shards
Possible values:
- 0 — Disabled (final query processing is done on the initiator node).
- 1 - Do not merge aggregation states from different servers for distributed query processing (query completelly processed on the shard, initiator only proxy the data), can be used in case it is for certain that there are different keys on different shards.
- 2 - Same as
1
but appliesORDER BY
andLIMIT
(it is not possible when the query processed completelly on the remote node, like fordistributed_group_by_no_merge=1
) on the initiator (can be used for queries withORDER BY
and/orLIMIT
).
Example
SELECT *
FROM remote('127.0.0.{2,3}', system.one)
GROUP BY dummy
LIMIT 1
SETTINGS distributed_group_by_no_merge = 1
FORMAT PrettyCompactMonoBlock
┌─dummy─┐
│ 0 │
│ 0 │
└───────┘
SELECT *
FROM remote('127.0.0.{2,3}', system.one)
GROUP BY dummy
LIMIT 1
SETTINGS distributed_group_by_no_merge = 2
FORMAT PrettyCompactMonoBlock
┌─dummy─┐
│ 0 │
└───────┘
Default value: 0
distributed_push_down_limit (#distributed-push-down-limit}
LIMIT will be applied on each shard separatelly. Usually you don't need to use it, since this will be done automatically if it is possible, i.e. for simple query SELECT FROM LIMIT.
Possible values:
- 0 - Disabled
- 1 - Enabled
Note
That with this setting the result of the query may be inaccurate.
optimize_skip_unused_shards_limit
Limit for number of sharding key values, turns off optimize_skip_unused_shards
if the limit is reached.
Too many values may require significant amount for processing, while the benefit is doubtful, since if you have huge number of values in IN (...)
, then most likely the query will be sent to all shards anyway.
Default value: 1000
optimize_skip_unused_shards
Enables or disables skipping of unused shards for SELECT queries that have sharding key condition in WHERE/PREWHERE
(assuming that the data is distributed by sharding key, otherwise does nothing).
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0
optimize_skip_unused_shards_rewrite_in
Rewrite IN in query for remote shards to exclude values that does not belong to the shard (requires optimize_skip_unused_shards).
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 1 (since it requires optimize_skip_unused_shards
anyway, which 0
by default)
allow_nondeterministic_optimize_skip_unused_shards
Allow nondeterministic (like rand
or dictGet
, since later has some caveats with updates) functions in sharding key.
Possible values:
- 0 — Disallowed.
- 1 — Allowed.
Default value: 0
optimize_skip_unused_shards_nesting
Controls optimize_skip_unused_shards
(hence still requires optimize_skip_unused_shards
) depends on the nesting level of the distributed query (case when you have Distributed
table that look into another Distributed
table).
Possible values:
- 0 — Disabled,
optimize_skip_unused_shards
works always. - 1 — Enables
optimize_skip_unused_shards
only for the first level. - 2 — Enables
optimize_skip_unused_shards
up to the second level.
Default value: 0
force_optimize_skip_unused_shards
Enables or disables query execution if optimize_skip_unused_shards is enabled and skipping of unused shards is not possible. If the skipping is not possible and the setting is enabled, an exception will be thrown.
Possible values:
- 0 — Disabled. ClickHouse does not throw an exception.
- 1 — Enabled. Query execution is disabled only if the table has a sharding key.
- 2 — Enabled. Query execution is disabled regardless of whether a sharding key is defined for the table.
Default value: 0
force_optimize_skip_unused_shards_nesting
Controls force_optimize_skip_unused_shards
(hence still requires force_optimize_skip_unused_shards
) depends on the nesting level of the distributed query (case when you have Distributed
table that look into another Distributed
table).
Possible values:
- 0 - Disabled,
force_optimize_skip_unused_shards
works always. - 1 — Enables
force_optimize_skip_unused_shards
only for the first level. - 2 — Enables
force_optimize_skip_unused_shards
up to the second level.
Default value: 0
optimize_distributed_group_by_sharding_key
Optimize GROUP BY sharding_key
queries, by avoiding costly aggregation on the initiator server (which will reduce memory usage for the query on the initiator server).
The following types of queries are supported (and all combinations of them):
SELECT DISTINCT [..., ]sharding_key[, ...] FROM dist
SELECT ... FROM dist GROUP BY sharding_key[, ...]
SELECT ... FROM dist GROUP BY sharding_key[, ...] ORDER BY x
SELECT ... FROM dist GROUP BY sharding_key[, ...] LIMIT 1
SELECT ... FROM dist GROUP BY sharding_key[, ...] LIMIT 1 BY x
The following types of queries are not supported (support for some of them may be added later):
SELECT ... GROUP BY sharding_key[, ...] WITH TOTALS
SELECT ... GROUP BY sharding_key[, ...] WITH ROLLUP
SELECT ... GROUP BY sharding_key[, ...] WITH CUBE
SELECT ... GROUP BY sharding_key[, ...] SETTINGS extremes=1
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0
See also:
Note
Right now it requires optimize_skip_unused_shards
(the reason behind this is that one day it may be enabled by default, and it will work correctly only if data was inserted via Distributed table, i.e. data is distributed according to sharding_key).
optimize_throw_if_noop
Enables or disables throwing an exception if an OPTIMIZE query didn’t perform a merge.
By default, OPTIMIZE
returns successfully even if it didn’t do anything. This setting lets you differentiate these situations and get the reason in an exception message.
Possible values:
- 1 — Throwing an exception is enabled.
- 0 — Throwing an exception is disabled.
Default value: 0.
optimize_functions_to_subcolumns
Enables or disables optimization by transforming some functions to reading subcolumns. This reduces the amount of data to read.
These functions can be transformed:
- length to read the size0 subcolumn.
- empty to read the size0 subcolumn.
- notEmpty to read the size0 subcolumn.
- isNull to read the null subcolumn.
- isNotNull to read the null subcolumn.
- count to read the null subcolumn.
- mapKeys to read the keys subcolumn.
- mapValues to read the values subcolumn.
Possible values:
- 0 — Optimization disabled.
- 1 — Optimization enabled.
Default value: 0
.
distributed_replica_error_half_life
- Type: seconds
- Default value: 60 seconds
Controls how fast errors in distributed tables are zeroed. If a replica is unavailable for some time, accumulates 5 errors, and distributed_replica_error_half_life is set to 1 second, then the replica is considered normal 3 seconds after the last error.
See also:
- load_balancing
- Table engine Distributed
- distributed_replica_error_cap
- distributed_replica_max_ignored_errors
distributed_replica_error_cap
- Type: unsigned int
- Default value: 1000
The error count of each replica is capped at this value, preventing a single replica from accumulating too many errors.
See also:
- load_balancing
- Table engine Distributed
- distributed_replica_error_half_life
- distributed_replica_max_ignored_errors
distributed_replica_max_ignored_errors
- Type: unsigned int
- Default value: 0
The number of errors that will be ignored while choosing replicas (according to load_balancing
algorithm).
See also:
- load_balancing
- Table engine Distributed
- distributed_replica_error_cap
- distributed_replica_error_half_life
distributed_directory_monitor_sleep_time_ms
Base interval for the Distributed table engine to send data. The actual interval grows exponentially in the event of errors.
Possible values:
- A positive integer number of milliseconds.
Default value: 100 milliseconds.
distributed_directory_monitor_max_sleep_time_ms
Maximum interval for the Distributed table engine to send data. Limits exponential growth of the interval set in the distributed_directory_monitor_sleep_time_ms setting.
Possible values:
- A positive integer number of milliseconds.
Default value: 30000 milliseconds (30 seconds).
distributed_directory_monitor_batch_inserts
Enables/disables inserted data sending in batches.
When batch sending is enabled, the Distributed table engine tries to send multiple files of inserted data in one operation instead of sending them separately. Batch sending improves cluster performance by better-utilizing server and network resources.
Possible values:
- 1 — Enabled.
- 0 — Disabled.
Default value: 0.
distributed_directory_monitor_split_batch_on_failure
Enables/disables splitting batches on failures.
Sometimes sending particular batch to the remote shard may fail, because of some complex pipeline after (i.e. MATERIALIZED VIEW
with GROUP BY
) due to Memory limit exceeded
or similar errors. In this case, retrying will not help (and this will stuck distributed sends for the table) but sending files from that batch one by one may succeed INSERT.
So installing this setting to 1
will disable batching for such batches (i.e. temporary disables distributed_directory_monitor_batch_inserts
for failed batches).
Possible values:
- 1 — Enabled.
- 0 — Disabled.
Default value: 0.
Note
This setting also affects broken batches (that may appears because of abnormal server (machine) termination and no fsync_after_insert
/fsync_directories
for Distributed table engine).
Warning
You should not rely on automatic batch splitting, since this may hurt performance.
os_thread_priority
Sets the priority (nice) for threads that execute queries. The OS scheduler considers this priority when choosing the next thread to run on each available CPU core.
Warning
To use this setting, you need to set the CAP_SYS_NICE
capability. The clickhouse-server
package sets it up during installation. Some virtual environments do not allow you to set the CAP_SYS_NICE
capability. In this case, clickhouse-server
shows a message about it at the start.
Possible values:
- You can set values in the range
[-20, 19]
.
Lower values mean higher priority. Threads with low nice
priority values are executed more frequently than threads with high values. High values are preferable for long-running non-interactive queries because it allows them to quickly give up resources in favour of short interactive queries when they arrive.
Default value: 0.
query_profiler_real_time_period_ns
Sets the period for a real clock timer of the query profiler. Real clock timer counts wall-clock time.
Possible values:
-
Positive integer number, in nanoseconds.
Recommended values:
- 10000000 (100 times a second) nanoseconds and less for single queries. -<PHONE_NUMBER> (once a second) for cluster-wide profiling.
-
0 for turning off the timer.
Type: UInt64.
Default value:<PHONE_NUMBER> nanoseconds (once a second).
See also:
- System table trace_log
query_profiler_cpu_time_period_ns
Sets the period for a CPU clock timer of the query profiler. This timer counts only CPU time.
Possible values:
-
A positive integer number of nanoseconds.
Recommended values:
- 10000000 (100 times a second) nanoseconds and more for single queries. -<PHONE_NUMBER> (once a second) for cluster-wide profiling.
-
0 for turning off the timer.
Type: UInt64.
Default value:<PHONE_NUMBER> nanoseconds.
See also:
- System table trace_log
allow_introspection_functions
Enables or disables introspections functions for query profiling.
Possible values:
- 1 — Introspection functions enabled.
- 0 — Introspection functions disabled.
Default value: 0.
See Also
- Sampling Query Profiler
- System table trace_log
input_format_parallel_parsing
Enables or disables order-preserving parallel parsing of data formats. Supported only for TSV, TKSV, CSV and JSONEachRow formats.
Possible values:
- 1 — Enabled.
- 0 — Disabled.
Default value: 0
.
output_format_parallel_formatting
Enables or disables parallel formatting of data formats. Supported only for TSV, TKSV, CSV and JSONEachRow formats.
Possible values:
- 1 — Enabled.
- 0 — Disabled.
Default value: 0
.
min_chunk_bytes_for_parallel_parsing
- Type: unsigned int
- Default value: 1 MiB
The minimum chunk size in bytes, which each thread will parse in parallel.
output_format_avro_codec
Sets the compression codec used for output Avro file.
Type: string
Possible values:
null
— No compressiondeflate
— Compress with Deflate (zlib)snappy
— Compress with Snappy
Default value: snappy
(if available) or deflate
.
output_format_avro_sync_interval
Sets minimum data size (in bytes) between synchronization markers for output Avro file.
Type: unsigned int
Possible values: 32 (32 bytes) -<PHONE_NUMBER> (1 GiB)
Default value: 32768 (32 KiB)
output_format_avro_string_column_pattern
Regexp of column names of type String to output as Avro string
(default is bytes
).
RE2 syntax is supported.
Type: string
format_avro_schema_registry_url
Sets Confluent Schema Registry URL to use with AvroConfluent format.
Default value: Empty
.
input_format_avro_allow_missing_fields
Enables using fields that are not specified in Avro or AvroConfluent format schema. When a field is not found in the schema, ClickHouse uses the default value instead of throwing an exception.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0.
background_pool_size
Sets the number of threads performing background operations in table engines (for example, merges in MergeTree engine tables). This setting is applied from the default
profile at the ClickHouse server start and can’t be changed in a user session. By adjusting this setting, you manage CPU and disk load. Smaller pool size utilizes less CPU and disk resources, but background processes advance slower which might eventually impact query performance.
Before changing it, please also take a look at related MergeTree settings, such as number_of_free_entries_in_pool_to_lower_max_size_of_merge
and number_of_free_entries_in_pool_to_execute_mutation
.
Possible values:
- Any positive integer.
Default value: 16.
merge_selecting_sleep_ms
Sleep time for merge selecting when no part is selected. A lower setting triggers selecting tasks in background_schedule_pool
frequently, which results in a large number of requests to Zookeeper in large-scale clusters.
Possible values:
- Any positive integer.
Default value: 5000
.
parallel_distributed_insert_select
Enables parallel distributed INSERT ... SELECT
query.
If we execute INSERT INTO distributed_table_a SELECT ... FROM distributed_table_b
queries and both tables use the same cluster, and both tables are either replicated or non-replicated, then this query is processed locally on every shard.
Possible values:
- 0 — Disabled.
- 1 —
SELECT
will be executed on each shard from the underlying table of the distributed engine. - 2 —
SELECT
andINSERT
will be executed on each shard from/to the underlying table of the distributed engine.
Default value: 0.
insert_distributed_sync
Enables or disables synchronous data insertion into a Distributed table.
By default, when inserting data into a Distributed
table, the ClickHouse server sends data to cluster nodes in asynchronous mode. When insert_distributed_sync=1
, the data is processed synchronously, and the INSERT
operation succeeds only after all the data is saved on all shards (at least one replica for each shard if internal_replication
is true).
Possible values:
- 0 — Data is inserted in asynchronous mode.
- 1 — Data is inserted in synchronous mode.
Default value: 0
.
See Also
insert_distributed_one_random_shard
Enables or disables random shard insertion into a Distributed table when there is no distributed key.
By default, when inserting data into a Distributed
table with more than one shard, the ClickHouse server will reject any insertion request if there is no distributed key. When insert_distributed_one_random_shard = 1
, insertions are allowed and data is forwarded randomly among all shards.
Possible values:
- 0 — Insertion is rejected if there are multiple shards and no distributed key is given.
- 1 — Insertion is done randomly among all available shards when no distributed key is given.
Default value: 0
.
insert_shard_id
If not 0
, specifies the shard of Distributed table into which the data will be inserted synchronously.
If insert_shard_id
value is incorrect, the server will throw an exception.
To get the number of shards on requested_cluster
, you can check server config or use this query:
SELECT uniq(shard_num) FROM system.clusters WHERE cluster = 'requested_cluster';
Possible values:
- 0 — Disabled.
- Any number from
1
toshards_num
of corresponding Distributed table.
Default value: 0
.
Example
Query:
CREATE TABLE x AS system.numbers ENGINE = MergeTree ORDER BY number;
CREATE TABLE x_dist AS x ENGINE = Distributed('test_cluster_two_shards_localhost', currentDatabase(), x);
INSERT INTO x_dist SELECT * FROM numbers(5) SETTINGS insert_shard_id = 1;
SELECT * FROM x_dist ORDER BY number ASC;
Result:
┌─number─┐
│ 0 │
│ 0 │
│ 1 │
│ 1 │
│ 2 │
│ 2 │
│ 3 │
│ 3 │
│ 4 │
│ 4 │
└────────┘
use_compact_format_in_distributed_parts_names
Uses compact format for storing blocks for async (insert_distributed_sync
) INSERT into tables with Distributed
engine.
Possible values:
- 0 — Uses
user[:password]@host:port#default_database
directory format. - 1 — Uses
[shard{shard_index}[_replica{replica_index}]]
directory format.
Default value: 1
.
Note
- with
use_compact_format_in_distributed_parts_names=0
changes from cluster definition will not be applied for async INSERT. - with
use_compact_format_in_distributed_parts_names=1
changing the order of the nodes in the cluster definition, will change theshard_index
/replica_index
so be aware.
background_buffer_flush_schedule_pool_size
Sets the number of threads performing background flush in Buffer-engine tables. This setting is applied at the ClickHouse server start and can’t be changed in a user session.
Possible values:
- Any positive integer.
Default value: 16.
background_move_pool_size
Sets the number of threads performing background moves of data parts for MergeTree-engine tables. This setting is applied at the ClickHouse server start and can’t be changed in a user session.
Possible values:
- Any positive integer.
Default value: 8.
background_schedule_pool_size
Sets the number of threads performing background tasks for replicated tables, Kafka streaming, DNS cache updates. This setting is applied at ClickHouse server start and can’t be changed in a user session.
Possible values:
- Any positive integer.
Default value: 128.
background_fetches_pool_size
Sets the number of threads performing background fetches for replicated tables. This setting is applied at the ClickHouse server start and can’t be changed in a user session. For production usage with frequent small insertions or slow ZooKeeper cluster is recommended to use default value.
Possible values:
- Any positive integer.
Default value: 8.
always_fetch_merged_part
Prohibits data parts merging in Replicated*MergeTree-engine tables.
When merging is prohibited, the replica never merges parts and always downloads merged parts from other replicas. If there is no required data yet, the replica waits for it. CPU and disk load on the replica server decreases, but the network load on the cluster increases. This setting can be useful on servers with relatively weak CPUs or slow disks, such as servers for backups storage.
Possible values:
- 0 —
Replicated*MergeTree
-engine tables merge data parts at the replica. - 1 —
Replicated*MergeTree
-engine tables do not merge data parts at the replica. The tables download merged data parts from other replicas.
Default value: 0.
See Also
background_distributed_schedule_pool_size
Sets the number of threads performing background tasks for distributed sends. This setting is applied at the ClickHouse server start and can’t be changed in a user session.
Possible values:
- Any positive integer.
Default value: 16.
background_message_broker_schedule_pool_size
Sets the number of threads performing background tasks for message streaming. This setting is applied at the ClickHouse server start and can’t be changed in a user session.
Possible values:
- Any positive integer.
Default value: 16.
See Also
validate_polygons
Enables or disables throwing an exception in the pointInPolygon function, if the polygon is self-intersecting or self-tangent.
Possible values:
- 0 — Throwing an exception is disabled.
pointInPolygon
accepts invalid polygons and returns possibly incorrect results for them. - 1 — Throwing an exception is enabled.
Default value: 1.
transform_null_in
Enables equality of NULL values for IN operator.
By default, NULL
values can’t be compared because NULL
means undefined value. Thus, comparison expr = NULL
must always return false
. With this setting NULL = NULL
returns true
for IN
operator.
Possible values:
- 0 — Comparison of
NULL
values inIN
operator returnsfalse
. - 1 — Comparison of
NULL
values inIN
operator returnstrue
.
Default value: 0.
Example
Consider the null_in
table:
┌──idx─┬─────i─┐
│ 1 │ 1 │
│ 2 │ NULL │
│ 3 │ 3 │
└──────┴───────┘
Query:
SELECT idx, i FROM null_in WHERE i IN (1, NULL) SETTINGS transform_null_in = 0;
Result:
┌──idx─┬────i─┐
│ 1 │ 1 │
└──────┴──────┘
Query:
SELECT idx, i FROM null_in WHERE i IN (1, NULL) SETTINGS transform_null_in = 1;
Result:
┌──idx─┬─────i─┐
│ 1 │ 1 │
│ 2 │ NULL │
└──────┴───────┘
See Also
low_cardinality_max_dictionary_size
Sets a maximum size in rows of a shared global dictionary for the LowCardinality data type that can be written to a storage file system. This setting prevents issues with RAM in case of unlimited dictionary growth. All the data that can’t be encoded due to maximum dictionary size limitation ClickHouse writes in an ordinary method.
Possible values:
- Any positive integer.
Default value: 8192.
low_cardinality_use_single_dictionary_for_part
Turns on or turns off using of single dictionary for the data part.
By default, the ClickHouse server monitors the size of dictionaries and if a dictionary overflows then the server starts to write the next one. To prohibit creating several dictionaries set low_cardinality_use_single_dictionary_for_part = 1
.
Possible values:
- 1 — Creating several dictionaries for the data part is prohibited.
- 0 — Creating several dictionaries for the data part is not prohibited.
Default value: 0.
low_cardinality_allow_in_native_format
Allows or restricts using the LowCardinality data type with the Native format.
If usage of LowCardinality
is restricted, ClickHouse server converts LowCardinality
-columns to ordinary ones for SELECT
queries, and convert ordinary columns to LowCardinality
-columns for INSERT
queries.
This setting is required mainly for third-party clients which do not support LowCardinality
data type.
Possible values:
- 1 — Usage of
LowCardinality
is not restricted. - 0 — Usage of
LowCardinality
is restricted.
Default value: 1.
allow_suspicious_low_cardinality_types
Allows or restricts using LowCardinality with data types with fixed size of 8 bytes or less: numeric data types and FixedString(8_bytes_or_less)
.
For small fixed values using of LowCardinality
is usually inefficient, because ClickHouse stores a numeric index for each row. As a result:
- Disk space usage can rise.
- RAM consumption can be higher, depending on a dictionary size.
- Some functions can work slower due to extra coding/encoding operations.
Merge times in MergeTree-engine tables can grow due to all the reasons described above.
Possible values:
- 1 — Usage of
LowCardinality
is not restricted. - 0 — Usage of
LowCardinality
is restricted.
Default value: 0.
min_insert_block_size_rows_for_materialized_views
Sets the minimum number of rows in the block which can be inserted into a table by an INSERT
query. Smaller-sized blocks are squashed into bigger ones. This setting is applied only for blocks inserted into materialized view. By adjusting this setting, you control blocks squashing while pushing to materialized view and avoid excessive memory usage.
Possible values:
- Any positive integer.
- 0 — Squashing disabled.
Default value: 1048576.
See Also
min_insert_block_size_bytes_for_materialized_views
Sets the minimum number of bytes in the block which can be inserted into a table by an INSERT
query. Smaller-sized blocks are squashed into bigger ones. This setting is applied only for blocks inserted into materialized view. By adjusting this setting, you control blocks squashing while pushing to materialized view and avoid excessive memory usage.
Possible values:
- Any positive integer.
- 0 — Squashing disabled.
Default value: 268435456.
See also
output_format_pretty_grid_charset
Allows changing a charset which is used for printing grids borders. Available charsets are UTF-8, ASCII.
Example
SET output_format_pretty_grid_charset = 'UTF-8';
SELECT * FROM a;
┌─a─┐
│ 1 │
└───┘
SET output_format_pretty_grid_charset = 'ASCII';
SELECT * FROM a;
+-a-+
| 1 |
+---+
optimize_read_in_order
Enables ORDER BY optimization in SELECT queries for reading data from MergeTree tables.
Possible values:
- 0 —
ORDER BY
optimization is disabled. - 1 —
ORDER BY
optimization is enabled.
Default value: 1
.
See Also
optimize_aggregation_in_order
Enables GROUP BY optimization in SELECT queries for aggregating data in corresponding order in MergeTree tables.
Possible values:
- 0 —
GROUP BY
optimization is disabled. - 1 —
GROUP BY
optimization is enabled.
Default value: 0
.
See Also
mutations_sync
Allows to execute ALTER TABLE ... UPDATE|DELETE
queries (mutations) synchronously.
Possible values:
- 0 - Mutations execute asynchronously.
- 1 - The query waits for all mutations to complete on the current server.
- 2 - The query waits for all mutations to complete on all replicas (if they exist).
Default value: 0
.
See Also
ttl_only_drop_parts
Enables or disables complete dropping of data parts where all rows are expired in MergeTree tables.
When ttl_only_drop_parts
is disabled (by default), the ClickHouse server only deletes expired rows according to their TTL.
When ttl_only_drop_parts
is enabled, the ClickHouse server drops a whole part when all rows in it are expired.
Dropping whole parts instead of partial cleaning TTL-d rows allows having shorter merge_with_ttl_timeout
times and lower impact on system performance.
Possible values:
- 0 — The complete dropping of data parts is disabled.
- 1 — The complete dropping of data parts is enabled.
Default value: 0
.
See Also
- CREATE TABLE query clauses and settings (
merge_with_ttl_timeout
setting) - Table TTL
lock_acquire_timeout
Defines how many seconds a locking request waits before failing.
Locking timeout is used to protect from deadlocks while executing read/write operations with tables. When the timeout expires and the locking request fails, the ClickHouse server throws an exception "Locking attempt timed out! Possible deadlock avoided. Client should retry." with error code DEADLOCK_AVOIDED
.
Possible values:
- Positive integer (in seconds).
- 0 — No locking timeout.
Default value: 120
seconds.
cast_keep_nullable
Enables or disables keeping of the Nullable
data type in CAST operations.
When the setting is enabled and the argument of CAST
function is Nullable
, the result is also transformed to Nullable
type. When the setting is disabled, the result always has the destination type exactly.
Possible values:
- 0 — The
CAST
result has exactly the destination type specified. - 1 — If the argument type is
Nullable
, theCAST
result is transformed toNullable(DestinationDataType)
.
Default value: 0
.
Examples
The following query results in the destination data type exactly:
SET cast_keep_nullable = 0;
SELECT CAST(toNullable(toInt32(0)) AS Int32) as x, toTypeName(x);
Result:
┌─x─┬─toTypeName(CAST(toNullable(toInt32(0)), 'Int32'))─┐
│ 0 │ Int32 │
└───┴───────────────────────────────────────────────────┘
The following query results in the Nullable
modification on the destination data type:
SET cast_keep_nullable = 1;
SELECT CAST(toNullable(toInt32(0)) AS Int32) as x, toTypeName(x);
Result:
┌─x─┬─toTypeName(CAST(toNullable(toInt32(0)), 'Int32'))─┐
│ 0 │ Nullable(Int32) │
└───┴───────────────────────────────────────────────────┘
See Also
- CAST function
output_format_pretty_max_value_width
Limits the width of value displayed in Pretty formats. If the value width exceeds the limit, the value is cut.
Possible values:
- Positive integer.
- 0 — The value is cut completely.
Default value: 10000
symbols.
Examples
Query:
SET output_format_pretty_max_value_width = 10;
SELECT range(number) FROM system.numbers LIMIT 10 FORMAT PrettyCompactNoEscapes;
Result:
┌─range(number)─┐
│ [] │
│ [0] │
│ [0,1] │
│ [0,1,2] │
│ [0,1,2,3] │
│ [0,1,2,3,4⋯ │
│ [0,1,2,3,4⋯ │
│ [0,1,2,3,4⋯ │
│ [0,1,2,3,4⋯ │
│ [0,1,2,3,4⋯ │
└───────────────┘
Query with zero width:
SET output_format_pretty_max_value_width = 0;
SELECT range(number) FROM system.numbers LIMIT 5 FORMAT PrettyCompactNoEscapes;
Result:
┌─range(number)─┐
│ ⋯ │
│ ⋯ │
│ ⋯ │
│ ⋯ │
│ ⋯ │
└───────────────┘
output_format_pretty_row_numbers
Adds row numbers to output in the Pretty format.
Possible values:
- 0 — Output without row numbers.
- 1 — Output with row numbers.
Default value: 0
.
Example
Query:
SET output_format_pretty_row_numbers = 1;
SELECT TOP 3 name, value FROM system.settings;
Result:
┌─name────────────────────┬─value───┐
1. │ min_compress_block_size │ 65536 │
2. │ max_compress_block_size │ 1048576 │
3. │ max_block_size │ 65505 │
└─────────────────────────┴─────────┘
system_events_show_zero_values
Allows to select zero-valued events from system.events
.
Some monitoring systems require passing all the metrics values to them for each checkpoint, even if the metric value is zero.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0
.
Examples
Query
SELECT * FROM system.events WHERE event='QueryMemoryLimitExceeded';
Result
Ok.
Query
SET system_events_show_zero_values = 1;
SELECT * FROM system.events WHERE event='QueryMemoryLimitExceeded';
Result
┌─event────────────────────┬─value─┬─description───────────────────────────────────────────┐
│ QueryMemoryLimitExceeded │ 0 │ Number of times when memory limit exceeded for query. │
└──────────────────────────┴───────┴───────────────────────────────────────────────────────┘
persistent
Disables persistency for the Set and Join table engines.
Reduces the I/O overhead. Suitable for scenarios that pursue performance and do not require persistence.
Possible values:
- 1 — Enabled.
- 0 — Disabled.
Default value: 1
.
output_format_csv_null_representation
Defines the representation of NULL
for CSV output format. User can set any string as a value, for example, My NULL
.
Default value: \N
.
Examples
Query
SELECT * from csv_custom_null FORMAT CSV;
Result
788
\N
\N
Query
SET output_format_csv_null_representation = 'My NULL';
SELECT * FROM csv_custom_null FORMAT CSV;
Result
788
My NULL
My NULL
output_format_tsv_null_representation
Defines the representation of NULL
for TSV output format. User can set any string as a value, for example, My NULL
.
Default value: \N
.
Examples
Query
SELECT * FROM tsv_custom_null FORMAT TSV;
Result
788
\N
\N
Query
SET output_format_tsv_null_representation = 'My NULL';
SELECT * FROM tsv_custom_null FORMAT TSV;
Result
788
My NULL
My NULL
output_format_json_array_of_rows
Enables the ability to output all rows as a JSON array in the JSONEachRow format.
Possible values:
- 1 — ClickHouse outputs all rows as an array, each row in the
JSONEachRow
format. - 0 — ClickHouse outputs each row separately in the
JSONEachRow
format.
Default value: 0
.
Example of a query with the enabled setting
Query:
SET output_format_json_array_of_rows = 1;
SELECT number FROM numbers(3) FORMAT JSONEachRow;
Result:
[
{"number":"0"},
{"number":"1"},
{"number":"2"}
]
Example of a query with the disabled setting
Query:
SET output_format_json_array_of_rows = 0;
SELECT number FROM numbers(3) FORMAT JSONEachRow;
Result:
{"number":"0"}
{"number":"1"}
{"number":"2"}
allow_nullable_key
Allows using of the Nullable-typed values in a sorting and a primary key for MergeTree tables.
Possible values:
- 1 —
Nullable
-type expressions are allowed in keys. - 0 —
Nullable
-type expressions are not allowed in keys.
Default value: 0
.
aggregate_functions_null_for_empty
Enables or disables rewriting all aggregate functions in a query, adding -OrNull suffix to them. Enable it for SQL standard compatibility.
It is implemented via query rewrite (similar to count_distinct_implementation setting) to get consistent results for distributed queries.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0.
Example
Consider the following query with aggregate functions:
SELECT SUM(-1), MAX(0) FROM system.one WHERE 0;
With aggregate_functions_null_for_empty = 0
it would produce:
┌─SUM(-1)─┬─MAX(0)─┐
│ 0 │ 0 │
└─────────┴────────┘
With aggregate_functions_null_for_empty = 1
the result would be:
┌─SUMOrNull(-1)─┬─MAXOrNull(0)─┐
│ NULL │ NULL │
└───────────────┴──────────────┘
union_default_mode
Sets a mode for combining SELECT
query results. The setting is only used when shared with UNION without explicitly specifying the UNION ALL
or UNION DISTINCT
.
Possible values:
'DISTINCT'
— ClickHouse outputs rows as a result of combining queries removing duplicate rows.'ALL'
— ClickHouse outputs all rows as a result of combining queries including duplicate rows.''
— ClickHouse generates an exception when used withUNION
.
Default value: ''
.
See examples in UNION.
data_type_default_nullable
Allows data types without explicit modifiers NULL or NOT NULL in column definition will be Nullable.
Possible values:
- 1 — The data types in column definitions are set to
Nullable
by default. - 0 — The data types in column definitions are set to not
Nullable
by default.
Default value: 0
.
execute_merges_on_single_replica_time_threshold
Enables special logic to perform merges on replicas.
Possible values:
- Positive integer (in seconds).
- 0 — Special merges logic is not used. Merges happen in the usual way on all the replicas.
Default value: 0
.
Usage
Selects one replica to perform the merge on. Sets the time threshold from the start of the merge. Other replicas wait for the merge to finish, then download the result. If the time threshold passes and the selected replica does not perform the merge, then the merge is performed on other replicas as usual.
High values for that threshold may lead to replication delays.
It can be useful when merges are CPU bounded not IO bounded (performing heavy data compression, calculating aggregate functions or default expressions that require a large amount of calculations, or just very high number of tiny merges).
max_final_threads
Sets the maximum number of parallel threads for the SELECT
query data read phase with the FINAL modifier.
Possible values:
- Positive integer.
- 0 or 1 — Disabled.
SELECT
queries are executed in a single thread.
Default value: 16
.
opentelemetry_start_trace_probability
Sets the probability that the ClickHouse can start a trace for executed queries (if no parent trace context is supplied).
Possible values:
- 0 — The trace for all executed queries is disabled (if no parent trace context is supplied).
- Positive floating-point number in the range [0..1]. For example, if the setting value is
0,5
, ClickHouse can start a trace on average for half of the queries. - 1 — The trace for all executed queries is enabled.
Default value: 0
.
optimize_on_insert
Enables or disables data transformation before the insertion, as if merge was done on this block (according to table engine).
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 1.
Example
The difference between enabled and disabled:
Query:
SET optimize_on_insert = 1;
CREATE TABLE test1 (`FirstTable` UInt32) ENGINE = ReplacingMergeTree ORDER BY FirstTable;
INSERT INTO test1 SELECT number % 2 FROM numbers(5);
SELECT * FROM test1;
SET optimize_on_insert = 0;
CREATE TABLE test2 (`SecondTable` UInt32) ENGINE = ReplacingMergeTree ORDER BY SecondTable;
INSERT INTO test2 SELECT number % 2 FROM numbers(5);
SELECT * FROM test2;
Result:
┌─FirstTable─┐
│ 0 │
│ 1 │
└────────────┘
┌─SecondTable─┐
│ 0 │
│ 0 │
│ 0 │
│ 1 │
│ 1 │
└─────────────┘
Note that this setting influences Materialized view and MaterializedMySQL behaviour.
engine_file_empty_if_not_exists
Allows to select data from a file engine table without file.
Possible values:
- 0 — SELECT
throws exception.
- 1 — SELECT
returns empty result.
Default value: 0
.
engine_file_truncate_on_insert
Enables or disables truncate before insert in File engine tables.
Possible values:
- 0 — INSERT
query appends new data to the end of the file.
- 1 — INSERT
replaces existing content of the file with the new data.
Default value: 0
.
allow_experimental_geo_types
Allows working with experimental geo data types.
Possible values:
- 0 — Working with geo data types is disabled.
- 1 — Working with geo data types is enabled.
Default value: 0
.
database_atomic_wait_for_drop_and_detach_synchronously
Adds a modifier SYNC
to all DROP
and DETACH
queries.
Possible values:
- 0 — Queries will be executed with delay.
- 1 — Queries will be executed without delay.
Default value: 0
.
show_table_uuid_in_table_create_query_if_not_nil
Sets the SHOW TABLE
query display.
Possible values:
- 0 — The query will be displayed without table UUID.
- 1 — The query will be displayed with table UUID.
Default value: 0
.
allow_experimental_live_view
Allows creation of experimental live views.
Possible values:
- 0 — Working with live views is disabled.
- 1 — Working with live views is enabled.
Default value: 0
.
live_view_heartbeat_interval
Sets the heartbeat interval in seconds to indicate live view is alive .
Default value: 15
.
max_live_view_insert_blocks_before_refresh
Sets the maximum number of inserted blocks after which mergeable blocks are dropped and query for live view is re-executed.
Default value: 64
.
temporary_live_view_timeout
Sets the interval in seconds after which live view with timeout is deleted.
Default value: 5
.
periodic_live_view_refresh
Sets the interval in seconds after which periodically refreshed live view is forced to refresh.
Default value: 60
.
http_connection_timeout
HTTP connection timeout (in seconds).
Possible values:
- Any positive integer.
- 0 - Disabled (infinite timeout).
Default value: 1.
http_send_timeout
HTTP send timeout (in seconds).
Possible values:
- Any positive integer.
- 0 - Disabled (infinite timeout).
Default value: 1800.
http_receive_timeout
HTTP receive timeout (in seconds).
Possible values:
- Any positive integer.
- 0 - Disabled (infinite timeout).
Default value: 1800.
check_query_single_value_result
Defines the level of detail for the CHECK TABLE query result for MergeTree
family engines .
Possible values:
- 0 — the query shows a check status for every individual data part of a table.
- 1 — the query shows the general table check status.
Default value: 0
.
prefer_column_name_to_alias
Enables or disables using the original column names instead of aliases in query expressions and clauses. It especially matters when alias is the same as the column name, see Expression Aliases. Enable this setting to make aliases syntax rules in ClickHouse more compatible with most other database engines.
Possible values:
- 0 — The column name is substituted with the alias.
- 1 — The column name is not substituted with the alias.
Default value: 0
.
Example
The difference between enabled and disabled:
Query:
SET prefer_column_name_to_alias = 0;
SELECT avg(number) AS number, max(number) FROM numbers(10);
Result:
Received exception from server (version 21.5.1):
Code: 184. DB::Exception: Received from localhost:9000. DB::Exception: Aggregate function avg(number) is found inside another aggregate function in query: While processing avg(number) AS number.
Query:
SET prefer_column_name_to_alias = 1;
SELECT avg(number) AS number, max(number) FROM numbers(10);
Result:
┌─number─┬─max(number)─┐
│ 4.5 │ 9 │
└────────┴─────────────┘
limit
Sets the maximum number of rows to get from the query result. It adjusts the value set by the LIMIT clause, so that the limit, specified in the query, cannot exceed the limit, set by this setting.
Possible values:
- 0 — The number of rows is not limited.
- Positive integer.
Default value: 0
.
offset
Sets the number of rows to skip before starting to return rows from the query. It adjusts the offset set by the OFFSET clause, so that these two values are summarized.
Possible values:
- 0 — No rows are skipped .
- Positive integer.
Default value: 0
.
Example
Input table:
CREATE TABLE test (i UInt64) ENGINE = MergeTree() ORDER BY i;
INSERT INTO test SELECT number FROM numbers(500);
Query:
SET limit = 5;
SET offset = 7;
SELECT * FROM test LIMIT 10 OFFSET 100;
Result:
┌───i─┐
│ 107 │
│ 108 │
│ 109 │
└─────┘
optimize_syntax_fuse_functions
Enables to fuse aggregate functions with identical argument. It rewrites query contains at least two aggregate functions from sum, count or avg with identical argument to sumCount.
Possible values:
- 0 — Functions with identical argument are not fused.
- 1 — Functions with identical argument are fused.
Default value: 0
.
Example
Query:
CREATE TABLE fuse_tbl(a Int8, b Int8) Engine = Log;
SET optimize_syntax_fuse_functions = 1;
EXPLAIN SYNTAX SELECT sum(a), sum(b), count(b), avg(b) from fuse_tbl FORMAT TSV;
Result:
SELECT
sum(a),
sumCount(b).1,
sumCount(b).2,
(sumCount(b).1) / (sumCount(b).2)
FROM fuse_tbl
allow_experimental_database_replicated
Enables to create databases with Replicated engine.
Possible values:
- 0 — Disabled.
- 1 — Enabled.
Default value: 0
.
database_replicated_initial_query_timeout_sec
Sets how long initial DDL query should wait for Replicated database to precess previous DDL queue entries in seconds.
Possible values:
- Positive integer.
- 0 — Unlimited.
Default value: 300
.
distributed_ddl_task_timeout
Sets timeout for DDL query responses from all hosts in cluster. If a DDL request has not been performed on all hosts, a response will contain a timeout error and a request will be executed in an async mode. Negative value means infinite.
Possible values:
- Positive integer.
- 0 — Async mode.
- Negative integer — infinite timeout.
Default value: 180
.
distributed_ddl_output_mode
Sets format of distributed DDL query result.
Possible values:
throw
— Returns result set with query execution status for all hosts where query is finished. If query has failed on some hosts, then it will rethrow the first exception. If query is not finished yet on some hosts and distributed_ddl_task_timeout exceeded, then it throwsTIMEOUT_EXCEEDED
exception.none
— Is similar to throw, but distributed DDL query returns no result set.null_status_on_timeout
— ReturnsNULL
as execution status in some rows of result set instead of throwingTIMEOUT_EXCEEDED
if query is not finished on the corresponding hosts.never_throw
— Do not throwTIMEOUT_EXCEEDED
and do not rethrow exceptions if query has failed on some hosts.
Default value: throw
.
flatten_nested
Sets the data format of a nested columns.
Possible values:
- 1 — Nested column is flattened to separate arrays.
- 0 — Nested column stays a single array of tuples.
Default value: 1
.
Usage
If the setting is set to 0
, it is possible to use an arbitrary level of nesting.
Examples
Query:
SET flatten_nested = 1;
CREATE TABLE t_nest (`n` Nested(a UInt32, b UInt32)) ENGINE = MergeTree ORDER BY tuple();
SHOW CREATE TABLE t_nest;
Result:
┌─statement───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ CREATE TABLE default.t_nest
(
`n.a` Array(UInt32),
`n.b` Array(UInt32)
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS index_granularity = 8192 │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
Query:
SET flatten_nested = 0;
CREATE TABLE t_nest (`n` Nested(a UInt32, b UInt32)) ENGINE = MergeTree ORDER BY tuple();
SHOW CREATE TABLE t_nest;
Result:
┌─statement──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ CREATE TABLE default.t_nest
(
`n` Nested(a UInt32, b UInt32)
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS index_granularity = 8192 │
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
external_table_functions_use_nulls
Defines how mysql, postgresql and odbc] table functions use Nullable columns.
Possible values:
- 0 — The table function explicitly uses Nullable columns.
- 1 — The table function implicitly uses Nullable columns.
Default value: 1
.
Usage
If the setting is set to 0
, the table function does not make Nullable columns and inserts default values instead of NULL. This is also applicable for NULL values inside arrays.
output_format_arrow_low_cardinality_as_dictionary
Allows to convert the LowCardinality type to the DICTIONARY
type of the Arrow format for SELECT
queries.
Possible values:
- 0 — The
LowCardinality
type is not converted to theDICTIONARY
type. - 1 — The
LowCardinality
type is converted to theDICTIONARY
type.
Default value: 0
.
materialized_postgresql_max_block_size
Sets the number of rows collected in memory before flushing data into PostgreSQL database table.
Possible values:
- Positive integer.
Default value: 65536
.
materialized_postgresql_tables_list
Sets a comma-separated list of PostgreSQL database tables, which will be replicated via MaterializedPostgreSQL database engine.
Default value: empty list — means whole PostgreSQL database will be replicated.
materialized_postgresql_allow_automatic_update
Allow reloading table in the background, when schema changes are detected. DDL queries on the PostgreSQL side are not replicated via ClickHouse MaterializedPostgreSQL engine, because it is not allowed with PostgreSQL logical replication protocol, but the fact of DDL changes is detected transactionally. In this case, the default behaviour is to stop replicating those tables once DDL is detected. However, if this setting is enabled, then, instead of stopping the replication of those tables, they will be reloaded in the background via database snapshot without data losses and replication will continue for them.
Possible values:
- 0 — The table is not automatically updated in the background, when schema changes are detected.
- 1 — The table is automatically updated in the background, when schema changes are detected.
Default value: 0
.
materialized_postgresql_replication_slot
Allows to have user-managed replication slots. Must be used together with materialized_postgresql_snapshot
.
materialized_postgresql_replication_slot
A text string identifying a snapshot, from which initial dump of tables will be performed. Must be used together with materialized_postgresql_replication_slot
.
allow_experimental_projection_optimization
Enables or disables projection optimization when processing SELECT
queries.
Possible values:
- 0 — Projection optimization disabled.
- 1 — Projection optimization enabled.
Default value: 0
.
force_optimize_projection
Enables or disables the obligatory use of projections in SELECT
queries, when projection optimization is enabled (see allow_experimental_projection_optimization setting).
Possible values:
- 0 — Projection optimization is not obligatory.
- 1 — Projection optimization is obligatory.
Default value: 0
.
replication_alter_partitions_sync
Allows to set up waiting for actions to be executed on replicas by ALTER, OPTIMIZE or TRUNCATE queries.
Possible values:
- 0 — Do not wait.
- 1 — Wait for own execution.
- 2 — Wait for everyone.
Default value: 1
.
replication_wait_for_inactive_replica_timeout
Specifies how long (in seconds) to wait for inactive replicas to execute ALTER, OPTIMIZE or TRUNCATE queries.
Possible values:
- 0 — Do not wait.
- Negative integer — Wait for unlimited time.
- Positive integer — The number of seconds to wait.
Default value: 120
seconds.
regexp_max_matches_per_row
Sets the maximum number of matches for a single regular expression per row. Use it to protect against memory overload when using greedy regular expression in the extractAllGroupsHorizontal function.
Possible values:
- Positive integer.
Default value: 1000
.
log_queries_probability
Allows a user to write to query_log, query_thread_log, and query_views_log system tables only a sample of queries selected randomly with the specified probability. It helps to reduce the load with a large volume of queries in a second.
Possible values:
- 0 — Queries are not logged in the system tables.
- Positive floating-point number in the range [0..1]. For example, if the setting value is
0.5
, about half of the queries are logged in the system tables. - 1 — All queries are logged in the system tables.
Default value: 1
.
short_circuit_function_evaluation
Allows calculating the if, multiIf, and, and or functions according to a short scheme. This helps optimize the execution of complex expressions in these functions and prevent possible exceptions (such as division by zero when it is not expected).
Possible values:
enable
— Enables short-circuit function evaluation for functions that are suitable for it (can throw an exception or computationally heavy).force_enable
— Enables short-circuit function evaluation for all functions.disable
— Disables short-circuit function evaluation.
Default value: enable
.
max_hyperscan_regexp_length
Defines the maximum length for each regular expression in the hyperscan multi-match functions.
Possible values:
- Positive integer.
- 0 - The length is not limited.
Default value: 0
.
Example
Query:
SELECT multiMatchAny('abcd', ['ab','bcd','c','d']) SETTINGS max_hyperscan_regexp_length = 3;
Result:
┌─multiMatchAny('abcd', ['ab', 'bcd', 'c', 'd'])─┐
│ 1 │
└────────────────────────────────────────────────┘
Query:
SELECT multiMatchAny('abcd', ['ab','bcd','c','d']) SETTINGS max_hyperscan_regexp_length = 2;
Result:
Exception: Regexp length too large.
See Also
max_hyperscan_regexp_total_length
Sets the maximum length total of all regular expressions in each hyperscan multi-match function.
Possible values:
- Positive integer.
- 0 - The length is not limited.
Default value: 0
.
Example
Query:
SELECT multiMatchAny('abcd', ['a','b','c','d']) SETTINGS max_hyperscan_regexp_total_length = 5;
Result:
┌─multiMatchAny('abcd', ['a', 'b', 'c', 'd'])─┐
│ 1 │
└─────────────────────────────────────────────┘
Query:
SELECT multiMatchAny('abcd', ['ab','bc','c','d']) SETTINGS max_hyperscan_regexp_total_length = 5;
Result:
Exception: Total regexp lengths too large.
See Also