mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-09-21 09:10:48 +00:00
Update the description of FINAL
Задокументировал настройку max_final_threads и обновил описание запросов с FINAL.
This commit is contained in:
parent
8f2a830d83
commit
936020b381
@ -2474,7 +2474,6 @@ Possible values:
|
||||
|
||||
Default value: `0`.
|
||||
|
||||
|
||||
## aggregate_functions_null_for_empty {#aggregate_functions_null_for_empty}
|
||||
|
||||
Enables or disables rewriting all aggregate functions in a query, adding [-OrNull](../../sql-reference/aggregate-functions/combinators.md#agg-functions-combinator-ornull) suffix to them. Enable it for SQL standard compatibility.
|
||||
@ -2512,7 +2511,6 @@ With `aggregate_functions_null_for_empty = 1` the result would be:
|
||||
└───────────────┴──────────────┘
|
||||
```
|
||||
|
||||
|
||||
## union_default_mode {#union-default-mode}
|
||||
|
||||
Sets a mode for combining `SELECT` query results. The setting is only used when shared with [UNION](../../sql-reference/statements/select/union.md) without explicitly specifying the `UNION ALL` or `UNION DISTINCT`.
|
||||
@ -2527,7 +2525,6 @@ Default value: `''`.
|
||||
|
||||
See examples in [UNION](../../sql-reference/statements/select/union.md).
|
||||
|
||||
|
||||
## data_type_default_nullable {#data_type_default_nullable}
|
||||
|
||||
Allows data types without explicit modifiers [NULL or NOT NULL](../../sql-reference/statements/create/table.md#null-modifiers) in column definition will be [Nullable](../../sql-reference/data-types/nullable.md#data_type-nullable).
|
||||
@ -2539,7 +2536,6 @@ Possible values:
|
||||
|
||||
Default value: `0`.
|
||||
|
||||
|
||||
## execute_merges_on_single_replica_time_threshold {#execute-merges-on-single-replica-time-threshold}
|
||||
|
||||
Enables special logic to perform merges on replicas.
|
||||
@ -2559,4 +2555,15 @@ 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 {#max-final-threads}
|
||||
|
||||
Sets maximum number of threads to read from table with [FINAL](../../sql-reference\statements\select.md#select-from-final) modifier.
|
||||
|
||||
Possible values:
|
||||
|
||||
- Positive integer.
|
||||
- 0 or 1 — Disabled. Executed in a single thread.
|
||||
|
||||
Default value: `16`.
|
||||
|
||||
[Original article](https://clickhouse.tech/docs/en/operations/settings/settings/) <!-- hide -->
|
||||
|
@ -25,6 +25,8 @@ It is applicable when selecting data from tables that use the [MergeTree](../../
|
||||
- [Replicated](../../../engines/table-engines/mergetree-family/replication.md) versions of `MergeTree` engines.
|
||||
- [View](../../../engines/table-engines/special/view.md), [Buffer](../../../engines/table-engines/special/buffer.md), [Distributed](../../../engines/table-engines/special/distributed.md), and [MaterializedView](../../../engines/table-engines/special/materializedview.md) engines that operate over other engines, provided they were created over `MergeTree`-engine tables.
|
||||
|
||||
`SELECT` queries with `FINAL` are executed in parallel. The [max_final_threads](../../../operations/settings/settings.md#max-final-threads) setting limits the number of threads used.
|
||||
|
||||
### Drawbacks {#drawbacks}
|
||||
|
||||
Queries that use `FINAL` are executed slightly slower than similar queries that don’t, because:
|
||||
|
Loading…
Reference in New Issue
Block a user