*: only add default value for final aggregation to fix the aggregate push down (partition) union case (#35443)

[ti-srebot]

cherry-pick #35443 to release-5.3
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/35769

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/35769:release-5.3-d99b35822500

---

What problem does this PR solve?

Issue Number: close #35295

Problem Summary:

When table t is empty, this get empty result:

select avg(id) from t group by id;

While this get NULL:

select avg(id) from t;

We add NULL (default value) to the empty result here:

tidb/executor/aggregate.go

Lines 1368 to 1379 in 2de01b4

	if e.childResult.NumRows() == 0 {
	if !e.isChildReturnEmpty {
	err = e.appendResult2Chunk(chk)
	} else if e.defaultVal != nil {
	chk.Append(e.defaultVal, 0, 1)
	}
	e.executed = true
	return err
	}
	// Reach here, "e.childrenResults[0].NumRows() > 0" is guaranteed.
	e.isChildReturnEmpty = false
	e.inputRow = e.inputIter.Begin()

Whether or not add the NULL row depends on some conditions.

Note, for this case, the added NULL row make the final result wrong:

What is changed and how it works?

Only add NULL for final aggregation.
Otherwise the add NULL row operation would be done several times and send a wrong NULL row to the final aggregation.

• In aggregation push down optimze rule, I'll set the pushed down aggregation to non-final
• In physical plan, aggregation push down to cop phase, I'll not blindly change the upper aggregation to final/complete... because it's parent might be final and it's a partial itself
• In the executor builder phase, only set the default value for final aggregation

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

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[ti-srebot]

/run-all-tests

[ti-srebot]

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

[ti-chi-bot]

This pull request is closed because its related version has closed automatic cherry-picking.
If it's still needed, you can reopen it or just regenerate it using bot,
see:

https://prow.tidb.net/command-help#cherrypick
https://book.prow.tidb.net/#/plugins/cherrypicker