planner: fix unexpected decorrelate for LeftOuterJoin | tidb-test=pr/2589 (#63211)

[ti-chi-bot]

This is an automated cherry-pick of #63211

What problem does this PR solve?

Issue Number: close #62937

Problem Summary: For plan like the following, optimizer do decorrelate, it pulled up projection. But actually it shouldn't. Because the Apply is a LeftOuterJoin, and the projection, which is the right side of LeftOuterJoin, should output null when join condition is not matched. But this semantics will not be respected when the projection was pulled up. That's why we got wrong result.

TiDB [email protected]:test> explain SELECT (SELECT /*+ no_decorrelate()*/ IF(substr(dd.segment1,1,3)='600','X','') FROM dual WHERE dd.amount<>0) c1,dd.amount,dd.segment1 FROM test.hcst_test_sel_dual dd;
+---------------------------+---------+-----------+---------------+----------------------------------------------------------------------------+
| id                        | estRows | task      | access object | operator info                                                              |
+---------------------------+---------+-----------+---------------+----------------------------------------------------------------------------+
| Projection_9              | 3.00    | root      |               | Column#4, test.hcst_test_sel_dual.amount, test.hcst_test_sel_dual.segment1 |
| └─Apply_12                | 3.00    | root      |               | CARTESIAN left outer join, left side:TableReader_14                        |
|   ├─TableReader_14(Build) | 3.00    | root      |               | data:TableFullScan_13                                                      |
|   │ └─TableFullScan_13    | 3.00    | cop[tikv] | table:dd      | keep order:false, stats:pseudo                                             |
|   └─MaxOneRow_15(Probe)   | 3.00    | root      |               |                                                                            |
|     └─Projection_16       | 2.40    | root      |               | if(eq(substr(test.hcst_test_sel_dual.segment1, 1, 3), 600), X, )->Column#4 |
|       └─Selection_18      | 2.40    | root      |               | ne(test.hcst_test_sel_dual.amount, 0)                                      |
|         └─TableDual_20    | 3.00    | root      |               | rows:1                                                                     |
+---------------------------+---------+-----------+---------------+----------------------------------------------------------------------------+

What changed and how does it work?

Add a new function to check whether to do decorrelate for LeftOuterJoin. The basic idea is: if the expressions of projection are all from the outerPlan, it means the projection will not respect the semantics of LeftOuterJoin in most cases(unless the outerPlan output null row, and the expression is not null-rejective, this expression will also output null).
So when that happens, will not do decorrelate.

TODO:

1. The check in this PR is very conservative, it barely affects plan changes because it only blocks plans that are guaranteed to be wrong.
2. But from another perspective, this PR doesn’t fully solve the problem. For instance, we could block decorrelation when a projection expression involves inner plan columns and the expression is null-rejective. But our implementation of null-reject is buggy, so relying on it could cause new issues — that’s why I didn’t go down that path.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

@guo-shaoge This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

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