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system-variables: update docs for hint SET_VAR #15059

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Oct 31, 2023
Merged

system-variables: update docs for hint SET_VAR #15059

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Update system-variables.md
shawn0915 Oct 13, 2023
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hfxsd Oct 25, 2023
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hfxsd Oct 25, 2023
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Merge branch 'master' into sys-var-patch-1
Oreoxmt Oct 31, 2023
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16 changes: 8 additions & 8 deletions system-variables.md
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Expand Up @@ -672,7 +672,7 @@ This variable is an alias for [`last_insert_id`](#last_insert_id).

- Scope: SESSION | GLOBAL
- Persists to cluster: Yes
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Default value: `UNSPECIFIED`
- Value options: `NONE`, `FAST`, `HIGH_COMPRESSION`, `UNSPECIFIED`
- This variable is used to specify the data compression mode of the MPP Exchange operator. This variable takes effect when TiDB selects the MPP execution plan with the version number `1`. The meanings of the variable values are as follows:
Expand Down Expand Up @@ -1023,7 +1023,7 @@ mysql> SHOW GLOBAL VARIABLES LIKE 'max_prepared_stmt_count';

- Scope: SESSION | GLOBAL
- Persists to cluster: Yes
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Default value: ""
- This variable is used to specify a list of storage engines that might fall back to TiKV. If the execution of a SQL statement fails due to a failure of the specified storage engine in the list, TiDB retries executing this SQL statement with TiKV. This variable can be set to "" or "tiflash". When this variable is set to "tiflash", if TiFlash returns a timeout error (error code: ErrTiFlashServerTimeout), TiDB retries executing this SQL statement with TiKV.

Expand Down Expand Up @@ -2176,7 +2176,7 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;

- Scope: SESSION | GLOBAL
- Persists to cluster: Yes
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
- Type: Boolean
- Default value: `ON`
- TiDB v6.2.0 refactors the implementation of previous cost model. This variable controls whether to enable the refactored Cost Model implementation.
Expand Down Expand Up @@ -3192,14 +3192,14 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
> This variable is read-only for [TiDB Serverless](https://docs.pingcap.com/tidbcloud/select-cluster-tier#tidb-serverless).

- Scope: SESSION
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Default value: `tikv,tiflash,tidb`
- This variable is used to set the storage engine list that TiDB can use when reading data.

### tidb_last_ddl_info <span class="version-mark">New in v6.0.0</span>

- Scope: SESSION
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
- Default value: ""
- Type: String
- This is a read-only variable. It is internally used in TiDB to get the information of the last DDL operation within the current session.
Expand Down Expand Up @@ -5651,15 +5651,15 @@ For details, see [Identify Slow Queries](/identify-slow-queries.md).
### tiflash_fastscan <span class="version-mark">New in v6.3.0</span>

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Default value: `OFF`
- Type: Boolean
- If [FastScan](/tiflash/use-fastscan.md) is enabled (set to `ON`), TiFlash provides more efficient query performance, but does not guarantee the accuracy of the query results or data consistency.

### tiflash_fine_grained_shuffle_batch_size <span class="version-mark">New in v6.2.0</span>

- Scope: SESSION | GLOBAL
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Default value: `8192`
- Range: `[1, 18446744073709551615]`
- When Fine Grained Shuffle is enabled, the window function pushed down to TiFlash can be executed in parallel. This variable controls the batch size of the data sent by the sender.
Expand All @@ -5669,7 +5669,7 @@ For details, see [Identify Slow Queries](/identify-slow-queries.md).

- Scope: SESSION | GLOBAL
- Persists to cluster: Yes
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
- Type: Integer
- Default value: `0`
- Range: `[-1, 1024]`
Expand Down
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