This is a manual pick.

What problem does this PR solve?

Issue Number: close #48755

Problem Summary:

When UNIONALL's first child, it's possible that the selection is pushed at a position different with other children. This will cause inconsistent output columns among its children.

You can check the changed codes together with https://github.com/pingcap/tidb/blob/master/pkg/planner/core/rule_predicate_push_down.go#L479-L483 for more details.

And the failed case is something like Proj{c1}->UnionAll{c1, c2, c3}->(Select{c2 > 0, c3 > 0}->Proj{c1, c2, c3}, Proj{c1, c2, c3}->Select{c2>0, c3>0}).
UnionALL first only keeps the col c1. But its first child's selection also requires c2 and c3, so the projection of its first children will keep c2 and c3. The second child's projection doesn't need the c2 and c3 since the selection is pushed down.

What is changed and how it works?

We use the UNIONALL's self output columns to do the alignment.

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

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Fix the potential execution failure when UNION ALL's first child is TableDual