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Update the description of FINAL

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Задокументировал настройку max_final_threads и обновил описание запросов с FINAL.
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Dmitriy 2021-01-17 16:25:20 +03:00
parent 8f2a830d83
commit 936020b381
2 changed files with 13 additions and 4 deletions
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15
docs/en/operations/settings/settings.md
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@ -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 -->

2
docs/en/sql-reference/statements/select/from.md
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@ -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: