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

[ti-srebot]

cherry-pick #35443 to release-5.0
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/35766

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

git push git@github.com:ti-srebot/tidb.git pr/35766:release-5.0-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

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[ti-chi-bot]

@ti-srebot: PR needs rebase.

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[VelocityLight]

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