planner: Log warnings when agg function can not be pushdown in explain statement (#25553)

[ti-srebot]

cherry-pick #25553 to release-5.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25737

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---

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:
After #22713, if an expression can not be pushdown to storage layer, it will log a warning if current sql is an explain statement, however, for aggregation function agg(expr) if it can not be pushdown to storage layer becuase expr is not supported, no warning is logged because current implementation call AggFuncToPBExpr directly to check if the agg function can be pushed to storage layer.

What is changed and how it works?

Proposal: xxx

What's Changed:

Refine CheckAggCanPushCop and log warnings when agg function can not be pushdown in explain statement.
How it Works:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Side effects

• Performance regression
  • Consumes more CPU
  • Consumes more MEM
• Breaking backward compatibility

Release note

• Log warnings when agg function can not be pushdown in explain statement

[ti-srebot]

/run-all-tests

[ti-srebot]

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

[fzhedu]

/LGTM

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[REVIEW NOTIFICATION]

This pull request has been approved by:

• fzhedu
• hanfei1991

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[windtalker]

/merge

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/merge

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[fzhedu]

/merge

[ti-chi-bot]

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

Commit hash: 4499af9