diff --git a/optimizer-hints.md b/optimizer-hints.md index 1dcff3340ed72..b509c2243c715 100644 --- a/optimizer-hints.md +++ b/optimizer-hints.md @@ -928,3 +928,13 @@ CREATE TABLE t2 (a INT); EXPLAIN SELECT /*+ NO_HASH_JOIN(t1), NO_MERGE_JOIN(t1) */ * FROM t1, t2 WHERE t1.a=t2.a; ERROR 1815 (HY000): Internal : Can't find a proper physical plan for this query ``` + +- The system variable [`tidb_opt_enable_hash_join`](/system-variables.md#tidb_opt_enable_hash_join-new-in-v740) is set to `OFF`, and all other join types are also excluded. + +```sql +CREATE TABLE t1 (a INT); +CREATE TABLE t2 (a INT); +set tidb_opt_enable_hash_join=off; +EXPLAIN SELECT /*+ NO_MERGE_JOIN(t1) */ * FROM t1, t2 WHERE t1.a=t2.a; +ERROR 1815 (HY000): Internal : Can't find a proper physical plan for this query +``` diff --git a/system-variables.md b/system-variables.md index 927207fe5a4df..3230939834c6c 100644 --- a/system-variables.md +++ b/system-variables.md @@ -2958,6 +2958,79 @@ mysql> desc select count(distinct a) from test.t; - Default value: `ON` - This variable is used to control whether the optimizer estimates the number of rows based on column order correlation +<<<<<<< HEAD +======= +### tidb_opt_enable_hash_join New in v7.4.0 + +- Scope: SESSION | GLOBAL +- Persists to cluster: Yes +- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No +- Type: Boolean +- Default value: `ON` +- This variable is used to control whether the optimizer selects hash joins for tables. The value is `ON` by default. If it is set to `OFF`, the optimizer avoids selecting hash joins when generating execution plans, unless no other join algorithm is available. +- If both the system variable `tidb_opt_enable_hash_join` and the `HASH_JOIN` hint are configured, the `HASH_JOIN` hint takes precedence. Even if `tidb_opt_enable_hash_join` is set to `OFF`, when you specify a `HASH_JOIN` hint in a query, the TiDB optimizer still enforces a hash join plan. + +### tidb_opt_enable_non_eval_scalar_subquery New in v7.3.0 + +- Scope: SESSION | GLOBAL +- Persists to cluster: Yes +- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No +- Type: Boolean +- Default value: `OFF` +- This variable is used to control whether the `EXPLAIN` statement disables the execution of constant subqueries that can be expanded at the optimization stage. When this variable is set to `OFF`, the `EXPLAIN` statement expands the subquery in advance at the optimization stage. When this variable is set to `ON`, the `EXPLAIN` statement does not expand the subquery at the optimization stage. For more information, see [Disable subquery expansion](/explain-walkthrough.md#disable-the-early-execution-of-subqueries). + +### tidb_opt_enable_late_materialization New in v7.0.0 + +- Scope: SESSION | GLOBAL +- Persists to cluster: Yes +- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes +- Type: Boolean +- Default value: `ON` +- This variable is used to control whether to enable the [TiFlash late materialization](/tiflash/tiflash-late-materialization.md) feature. Note that TiFlash late materialization does not take effect in the [fast scan mode](/tiflash/use-fastscan.md). +- When this variable is set to `OFF` to disable the TiFlash late materialization feature, to process a `SELECT` statement with filter conditions (`WHERE` clause), TiFlash scans all the data of the required columns before filtering. When this variable is set to `ON` to enable the TiFlash late materialization feature, TiFlash can first scan the column data related to the filter conditions that are pushed down to the TableScan operator, filter the rows that meet the conditions, and then scan the data of other columns of these rows for further calculations, thereby reducing IO scans and computations of data processing. + +### tidb_opt_enable_mpp_shared_cte_execution New in v7.2.0 + +> **Warning:** +> +> The feature controlled by this variable is experimental. It is not recommended that you use it in the production environment. This feature might be changed or removed without prior notice. If you find a bug, you can report an [issue](https://github.com/pingcap/tidb/issues) on GitHub. + +- Scope: SESSION | GLOBAL +- Persists to cluster: Yes +- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes +- Type: Boolean +- Default value: `OFF` +- This variable controls whether the non-recursive [Common Table Expressions (CTE)](/sql-statements/sql-statement-with.md) can be executed on TiFlash MPP. By default, when this variable is disabled, CTE is executed on TiDB, which has a large performance gap compared with enabling this feature. + +### tidb_opt_fix_control New in v7.1.0 + + + +- Scope: SESSION | GLOBAL +- Persists to cluster: Yes +- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes +- Type: String +- Default value: `""` +- This variable is used to control some internal behaviors of the optimizer. +- The optimizer's behavior might vary depending on user scenarios or SQL statements. This variable provides a more fine-grained control over the optimizer and helps to prevent performance regression after upgrading caused by behavior changes in the optimizer. +- For a more detailed introduction, see [Optimizer Fix Controls](/optimizer-fix-controls.md). + + + + + +- Scope: SESSION | GLOBAL +- Persists to cluster: Yes +- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes +- Type: String +- Default value: `""` +- This variable is used to control some internal behaviors of the optimizer. +- The optimizer's behavior might vary depending on user scenarios or SQL statements. This variable provides a more fine-grained control over the optimizer and helps to prevent performance regression after upgrading caused by behavior changes in the optimizer. +- For a more detailed introduction, see [Optimizer Fix Controls](https://docs.pingcap.com/tidb/v7.2/optimizer-fix-controls). + + + +>>>>>>> b80e0c70b6 (sysvar: add system variable `tidb_opt_enable_hash_join` (#14905)) ### tidb_opt_force_inline_cte New in v6.3.0 - Scope: SESSION | GLOBAL