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doc: update the doc for hash_join_build and hash_join_probe hints #10451

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Sep 16, 2022
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doc: update the doc for hash_join_build and hash_join_probe hints #10451

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doc: update the doc for hash_join_build and hash_join_probe hints
shichun-0415 Sep 15, 2022
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TomShawn Sep 16, 2022
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16 changes: 16 additions & 0 deletions optimizer-hints.md
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Expand Up @@ -133,6 +133,22 @@ select /*+ HASH_JOIN(t1, t2) */ * from t1, t2 where t1.id = t2.id;
>
> `TIDB_HJ` is the alias for `HASH_JOIN` in TiDB 3.0.x and earlier versions. If you are using any of these versions, you must apply the `TIDB_HJ(t1_name [, tl_name ...])` syntax for the hint. For the later versions of TiDB, `TIDB_HJ` and `HASH_JOIN` are both valid names for the hint, but `HASH_JOIN` is recommended.

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

The `HASH_JOIN_BUILD(t1_name [, tl_name ...])` hint tells the optimizer to use the hash join algorithm on specified tables with these tables working as the build end. In this way, you can build hash tables using specific tables. For example:

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

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

The `HASH_JOIN_PROBE(t1_name [, tl_name ...])` hint tells the optimizer to use the hash join algorithm on specified tables with these tables working as the probe end. In this way, you can execute the hash join algorithm with specific tables as the probe end. For example:

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

### SEMI_JOIN_REWRITE()

The `SEMI_JOIN_REWRITE()` hint tells the optimizer to rewrite the semi-join query to an ordinary join query. Currently, this hint only works for `EXISTS` subqueries.
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