planner: fix bug that aggregate push down may generate wrong plan for outer joins (#34468)

[ti-srebot]

cherry-pick #34468 to release-5.2
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/34647

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git push git@github.com:ti-srebot/tidb.git pr/34647:release-5.2-43984d89e0e5

---

What problem does this PR solve?

Issue Number: close #34465

Problem Summary:
When we push down aggregate past outer join, the top count aggregate assumed
that the partial count aggregate always return non-null value. But in case of
outer joins when the aggregate is pushed down to the null generating side of
the outer join, the input for the final aggregate may be nullable.

Before this patch, the final count aggregate always assume its input is not
nullable, hence during aggregate elimination, the optimizer rewrote aggregate
into a plain column of null-generating side of outer join, which led to the
wrong result.

Check List

Tests

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

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Reminiscent
• qw4990

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[ti-srebot]

/run-all-tests

[ti-srebot]

@Reminiscent you're already a collaborator in bot's repo.

[fixdb]

/run-all-tests

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/6b74e86c2aadfcddc465e980e3b9f27afcb4a66b

[chrysan]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 82ec1d7

[fixdb]

/run-unit-test

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