Add the hint LIMIT_TO_COP() (#7202)

* add hint LIMIT_TO_COP() * update * Update optimizer-hints.md Co-authored-by: 虎 <[email protected]> * Update optimizer-hints.md * Update optimizer-hints.md Co-authored-by: TomShawn <[email protected]> Co-authored-by: 虎 <[email protected]>
pingcap · Sep 27, 2021 · 49472f9 · 49472f9
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commit 49472f9
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diff --git a/optimizer-hints.md b/optimizer-hints.md
@@ -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：
+
+
+{{< 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：