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Add the hint LIMIT_TO_COP() (#7202) #7210

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Sep 27, 2021
Merged

Add the hint LIMIT_TO_COP() (#7202) #7210

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add hint LIMIT_TO_COP()
qw4990 Sep 27, 2021
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update
qw4990 Sep 27, 2021
21e1cdd
Update optimizer-hints.md
TomShawn Sep 27, 2021
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Update optimizer-hints.md
TomShawn Sep 27, 2021
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Update optimizer-hints.md
TomShawn Sep 27, 2021
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TomShawn Sep 27, 2021
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11 changes: 11 additions & 0 deletions optimizer-hints.md
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Expand Up @@ -182,6 +182,17 @@ SELECT /*+ IGNORE_INDEX(t1, idx1, idx2) */ * FROM t t1;
SELECT /*+ AGG_TO_COP() */ sum(t1.a) FROM t t1;
```

### LIMIT_TO_COP()

`LIMIT_TO_COP()` 提示优化器将指定查询块中的 `Limit` 和 `TopN` 算子下推到 coprocessor。优化器没有下推 `Limit` 或者 `TopN` 算子时建议尝试使用该提示。例如：

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{{< copyable "sql" >}}

```sql
SELECT /*+ LIMIT_TO_COP() */ * FROM t WHERE a = 1 AND b > 10 ORDER BY c LIMIT 1;
```

### READ_FROM_STORAGE(TIFLASH[t1_name [, tl_name ...]], TIKV[t2_name [, tl_name ...]])

`READ_FROM_STORAGE(TIFLASH[t1_name [, tl_name ...]], TIKV[t2_name [, tl_name ...]])` 提示优化器从指定的存储引擎来读取指定的表，目前支持的存储引擎参数有 `TIKV` 和 `TIFLASH`。如果为表指定了别名，就只能使用表的别名作为 `READ_FROM_STORAGE()` 的参数；如果没有指定别名，则用表的本名作为其参数。例如：
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