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Add the hint LIMIT_TO_COP() #7202

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Sep 27, 2021
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Add the hint LIMIT_TO_COP() #7202

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add hint LIMIT_TO_COP()
qw4990 Sep 27, 2021
a30828c
update
qw4990 Sep 27, 2021
9f11d17
Update optimizer-hints.md
TomShawn Sep 27, 2021
5954ca5
Update optimizer-hints.md
TomShawn Sep 27, 2021
965e304
Update optimizer-hints.md
TomShawn Sep 27, 2021
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11 changes: 11 additions & 0 deletions optimizer-hints.md
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Expand Up @@ -183,6 +183,17 @@ 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()` 提示优化器将制定查询快中的 Limit 和 TopN 算子下推到 coprocessor。如果优化器没有下推 Limit 或者 TopN 算子，建议尝试。例如：
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{{< copyable "sql" >}}

```sql
select /*+ LIMIT_TO_COP() */ * from t where a = 1 and b > 10 and b < 20 and c > 50 order by d 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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