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planner: add the doc for hint #12873

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planner: add the doc for hint #12873

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planner: add the doc for hint
ran-huang Mar 13, 2023
c020151
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ran-huang Mar 20, 2023
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50 changes: 50 additions & 0 deletions optimizer-hints.md
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Expand Up @@ -201,6 +201,32 @@ EXPLAIN SELECT * FROM t WHERE EXISTS (SELECT /*+ SEMI_JOIN_REWRITE() */ 1 FROM t

From the preceding example, you can see that when using the `SEMI_JOIN_REWRITE()` hint, TiDB can select the execution method of IndexJoin based on the driving table `t1`.

### SHUFFLE_JOIN(t1_name [, tl_name ...])

The `SHUFFLE_JOIN(t1_name [, tl_name ...])` hint tells the optimizer to use the Shuffle Join algorithm on specified tables. This hint only takes effect in the MPP mode. For example:

```sql
SELECT /*+ SHUFFLE_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
```

> **Note:**
>
> - Before using this hint, make sure that the current TiDB cluster can support using TiFlash MPP mode in the query. For details, refer to [Use TiFlash MPP Mode](/tiflash/use-tiflash-mpp-mode.md).
> - This hint can be used in combination with the [`HASH_JOIN_BUILD` hint](#hash_join_buildt1_name--tl_name-) and [`HASH_JOIN_PROBE` hint](#hash_join_probet1_name--tl_name-) to control the Build side and Probe side of the Shuffle Join algorithm.

### BROADCAST_JOIN(t1_name [, tl_name ...])

`BROADCAST_JOIN(t1_name [, tl_name ...])` hint tells the optimizer to use the Broadcast Join algorithm on specified tables. This hint only takes effect in the MPP mode. For example:

```sql
SELECT /*+ BROADCAST_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
```

> **Note:**
>
> - Before using this hint, make sure that the current TiDB cluster can support using TiFlash MPP mode in the query. For details, refer to [Use TiFlash MPP Mode](/tiflash/use-tiflash-mpp-mode.md).
>- This hint can be used in combination with the [`HASH_JOIN_BUILD` hint](#hash_join_buildt1_name--tl_name-) and [`HASH_JOIN_PROBE` hint](#hash_join_probet1_name--tl_name-) to control the Build side and Probe side of the Broadcast Join algorithm.

### NO_DECORRELATE()

The `NO_DECORRELATE()` hint tells the optimizer not to try to perform decorrelation for the correlated subquery in the specified query block. This hint is applicable to the `EXISTS`, `IN`, `ANY`, `ALL`, `SOME` subqueries and scalar subqueries that contain correlated columns (that is, correlated subqueries).
Expand Down Expand Up @@ -287,6 +313,30 @@ The `STREAM_AGG()` hint tells the optimizer to use the stream aggregation algori
select /*+ STREAM_AGG() */ count(*) from t1, t2 where t1.a > 10 group by t1.id;
```

### MPP_1PHASE_AGG()

`MPP_1PHASE_AGG()` tells the optimizer to use the one-phase aggregation algorithm for all aggregate functions in the specified query block. This hint only takes effect in the MPP mode. For example:

```sql
SELECT /*+ MPP_1PHASE_AGG() */ COUNT(*) FROM t1, t2 WHERE t1.a > 10 GROUP BY t1.id;
```

> **Note:**
>
> - Before using this hint, make sure that the current TiDB cluster can support using TiFlash MPP mode in the query. For details, refer to [Use TiFlash MPP Mode](/tiflash/use-tiflash-mpp-mode.md).
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### MPP_2PHASE_AGG()

`MPP_2PHASE_AGG()` tells the optimizer to use the two-phase aggregation algorithm for all aggregate functions in the specified query block. This hint only takes effect in the MPP mode. For example:

```sql
SELECT /*+ MPP_2PHASE_AGG() */ COUNT(*) FROM t1, t2 WHERE t1.a > 10 GROUP BY t1.id;
```

> **Note:**
>
> Before using this hint, make sure that the current TiDB cluster can support using TiFlash MPP mode in the query. For details, refer to [Use TiFlash MPP Mode](/tiflash/use-tiflash-mpp-mode.md).

### USE_INDEX(t1_name, idx1_name [, idx2_name ...])

The `USE_INDEX(t1_name, idx1_name [, idx2_name ...])` hint tells the optimizer to use only the given index(es) for a specified `t1_name` table. For example, applying the following hint has the same effect as executing the `select * from t t1 use index(idx1, idx2);` statement.
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16 changes: 14 additions & 2 deletions system-variables.md
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Expand Up @@ -3010,6 +3010,18 @@ mysql> desc select count(distinct a) from test.t;
- Default value: `OFF`
- This variable is used to control whether common table expressions (CTEs) in the entire session are inlined or not. The default value is `OFF`, which means that inlining CTE is not enforced by default. However, you can still inline CTE by specifying the `MERGE()` hint. If the variable is set to `ON`, all CTEs (except recursive CTE) in this session are forced to be inlined.

### tidb_opt_advanced_join_hint <span class="version-mark">New in v7.0.0</span>

- Scope: SESSION | GLOBAL
- Persists to cluster: Yes
- Type: Boolean
- Default value: `ON`
- This variable is used to control whether the Join Method Hint, such as [`HASH_JOIN()` Hint](/optimizer-hints.md#hash_joint1_name--tl_name-) and [`MERGE_JOIN()` Hint](/optimizer-hints.md#merge_joint1_name--tl_name-), affects the Join Reorder optimization process, including the use of [`LEADING()` Hint](/optimizer-hints.md#leadingt1_name--tl_name-). The default value is `ON`, which means that it does not affect. If it is set to `OFF`, there may be conflicts in some scenarios where both Join Method Hint and `LEADING()` Hint are used at the same time.
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> **Note:**
>
> The behavior of versions earlier than v7.0.0 is consistent with that of setting this variable to `OFF`. To ensure forward compatibility, when you upgrade from an earlier version to a v7.0.0 or later cluster, this variable is set to `OFF`. To obtain more flexible Hint behavior, it is strongly recommended to switch this variable to `ON` under the condition that there is no performance regression.
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### tidb_opt_insubq_to_join_and_agg

- Scope: SESSION | GLOBAL
Expand Down Expand Up @@ -3876,7 +3888,7 @@ For details, see [Identify Slow Queries](/identify-slow-queries.md).
- Scope: GLOBAL
- Type: Boolean
- Default value: `OFF`
- This variable is read-only. It controls whether to enable [statements summary persistence](/statement-summary-tables.md#persist-statements-summary).
- This variable is read-only. It controls whether to enable [statements summary persistence](/statement-summary-tables.md#persist-statements-summary).

<CustomContent platform="tidb">

Expand All @@ -3901,7 +3913,7 @@ For details, see [Identify Slow Queries](/identify-slow-queries.md).
- Scope: GLOBAL
- Type: String
- Default value: `"tidb-statements.log"`
- This variable is read-only. It specifies the file to which persistent data is written when [statements summary persistence](/statement-summary-tables.md#persist-statements-summary) is enabled.
- This variable is read-only. It specifies the file to which persistent data is written when [statements summary persistence](/statement-summary-tables.md#persist-statements-summary) is enabled.

<CustomContent platform="tidb">

Expand Down