Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

optimizer-hints: add docs for nth_plan() hint #4227

Merged
merged 4 commits into from
Aug 24, 2020
Merged

optimizer-hints: add docs for nth_plan() hint #4227

Changes from 1 commit
Commits
Show all changes
4 commits
Select commit Hold shift + click to select a range
5f294c7
optimizer-hints: add docs for `nth_plan()` hint
time-and-fate Aug 7, 2020
be28e71
optimizer-hints: add a notes for `nth_plan()`
time-and-fate Aug 20, 2020
0c5deab
Merge branch 'master' into master
TomShawn Aug 24, 2020
7a209f7
Update optimizer-hints.md
TomShawn Aug 24, 2020
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
16 changes: 16 additions & 0 deletions optimizer-hints.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -313,3 +313,19 @@ SELECT /*+ READ_CONSISTENT_REPLICA() */ * FROM t;
```sql
prepare stmt FROM 'SELECT /*+ IGNORE_PLAN_CACHE() */ * FROM t WHERE t.id = ?';
```

### NTH_PLAN(N)

`NTH_PLAN(N)` 提示优化器选用在物理优化阶段搜索到的第 `N` 个物理计划。`N` 必须是正整数。

如果指定的 `N` 超出了物理优化阶段的搜索范围，TiDB 会返回 warning，并使用和没有该 Hint 时一样的策略选择最优物理计划。

该 Hint 在启用 cascades planner 的情况下不会生效。

以下示例会强制优化器在物理阶段选择搜索到的第 3 个物理计划：

{{< copyable "sql" >}}

```sql
SELECT /*+ NTH_PLAN(3) */ count(*) from t where a > 5;
```