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Settings

A session can change a setting with SET name = value when its scope is session. The scope server_runtime settings change with ALTER SETTING, and server_static settings change only in the configuration.

Query

Name	Type	Default	Scope	Allowed values	Description
`max_query_rows`	integer	1000000	session	1 to 1000000000	Maximum number of result rows returned by a single query.
`result_overflow_mode`	enumeration	throw	session	`throw`, `break`	Whether a query with more rows than max_query_rows fails (throw) or returns the first rows marked as truncated (break).
`max_execution_time_ms`	integer	0	session	0 to 86400000	Longest time one query may run, in milliseconds. 0 means no limit.
`max_rows_to_read`	integer	0	session	0 to 4503599627370496	Most rows one query may scan. 0 means no limit.
`max_result_rows`	integer	0	session	0 to 4503599627370496	Most rows one query may return before it fails. 0 means no limit.
`max_memory_bytes`	integer	0	session	0 to 4503599627370496	Most memory, in bytes, that one query may allocate. 0 means no limit.
`max_bytes_before_external_group_by`	integer	0	session	-1 to 4503599627370496	Size of aggregate state, in bytes, at which GROUP BY writes its groups to temporary files. 0 means half of max_memory_bytes, and -1 means never.
`max_bytes_before_external_sort`	integer	0	session	-1 to 4503599627370496	Size of buffered ORDER BY rows, in bytes, at which a sort writes sorted runs to temporary files. 0 means half of max_memory_bytes, and -1 means never.
`max_temp_space_bytes`	integer	8589934592	server_static	0 to 4503599627370496	Most disk space, in bytes, that the spill files of all queries may use together. 0 means no limit.
`max_concurrent_queries`	integer	0	server_runtime	0 to 1000000	Most queries that may run at once on the server. 0 means no limit.
`join_use_nulls`	boolean	true	session		A LEFT join fills unmatched right-side columns with NULL when this is true, and with the type's default value when it is false. A boolean setting also takes 1 and 0.
`asof_join_algorithm`	enumeration	auto	session	`auto`, `hash`, `merge`	Algorithm for ASOF joins. auto lets the planner choose, hash searches sorted vectors per key, and merge sorts both sides and walks them once.
`union_default_mode`	enumeration	empty	session	empty, `ALL`, `DISTINCT`	How a UNION without ALL or DISTINCT behaves. ALL keeps duplicate rows, DISTINCT drops them, and empty refuses the query, as ClickHouse does.
`aggregate_undefined_returns_nan`	boolean	false	session		Result of varSamp, stddevSamp, covarSamp and corr for a group with fewer than two rows. 0 gives NULL, and 1 gives nan as ClickHouse does.
`prefer_column_name_to_alias`	boolean	false	session		Which one wins when an identifier is both a SELECT alias and a column. 0 picks the alias, as ClickHouse does, and 1 picks the column.
`query_priority`	enumeration	normal	session	`high`, `normal`, `low`	Scheduling band of this session's queries when the CPU is busy. Only an administrator may choose high.

Distributed query

Name	Type	Default	Scope	Allowed values	Description
`distributed_timeout_ms`	integer	10000	session	0 to 3600000	Time limit for a whole distributed query, in milliseconds.
`distributed_distinct_max`	integer	1048576	session	1 to 1073741824	Most COUNT(DISTINCT) values kept per aggregate and fragment.
`distributed_consistency`	enumeration	full	session	`full`, `partial`	Default read consistency of a distributed query. full waits for every shard, and partial answers without a shard that is down or slow.

Tracing

Name	Type	Default	Scope	Allowed values	Description
`opentelemetry_start_trace_probability`	ratio	0	session		Chance from 0 to 1 that a query is traced whatever the query subsystem ratio.
`opentelemetry_trace_processors`	boolean	false	session		Accepted for ClickHouse compatibility and has no effect: every traced query records its operator spans.
`log_processors_profiles`	boolean	false	session		Writes one row per query operator to system.processors_profile_log.
`memory_profiler_step`	integer	0	session	0 to 1099511627776	Records a system.trace_log row each time a query's memory charges add up to this many bytes. 0 turns it off.
`memory_profiler_sample_probability`	ratio	0	session		Chance from 0 to 1 that one memory charge of a query is recorded in system.trace_log.

Storage

Name	Type	Default	Scope	Allowed values	Description
`flush_max_rows`	integer	1048576	server_runtime	0 to 17179869184	Flushes a table's segment once this many rows are unflushed. 0 disables it.
`flush_interval_ms`	integer	60000	server_runtime	0 to 86400000	Flushes a table's segment this many milliseconds after its previous flush. 0 disables it.

Experimental

Name	Type	Default	Scope	Allowed values	Description
`allow_experimental_arrays`	boolean	true	server_static		Whether array and nested-type columns are enabled. The value is fixed at build time and cannot be changed.

Search

Name	Type	Default	Scope	Allowed values	Description
`search_nprobe`	integer	8	session	1 to 65536	IVF cells probed per part by an approximate vector KNN query. A higher value gives better recall and a slower query, and it applies only when vector_search_approximate is 1.
`vector_search_approximate`	boolean	false	session		Whether KNN search uses the approximate IVF index. 0 compares every vector, and 1 uses the faster index, whose recall can be below 1.
`bm25_k1`	string	1.2	session		BM25 term-frequency saturation k1, as decimal text. The standard value is 1.2.
`bm25_b`	string	0.75	session		BM25 document-length normalization b between 0 and 1, as decimal text. The standard value is 0.75.

Retention

Name	Type	Default	Scope	Allowed values	Description
`allow_partial_below_drop_horizon`	boolean	false	session		Returns partial rows with a partial-consistency signal, instead of refusing a query that reaches below the raw-drop horizon and cannot use a rollup.