expression: Add warning info for exprs that can not be pushed to storage layer (#22713)

[ti-srebot]

cherry-pick #22713 to release-4.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/23020

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git push [email protected]:ti-srebot/tidb.git pr/23020:release-4.0-3ad7f40140cd

---

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

Currently, when user running a query, and found the query is slower than expected because some part of the query(e.g. Selection or Aggregation) is not pushed down to coprocessor, there is no easy way to find out which function not supported by coprocessor.

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:

According to Mysql document:

The EXPLAIN statement produces extra (“extended”) information that is not part of EXPLAIN output but can be viewed by issuing a SHOW WARNINGS statement following EXPLAIN. As of MySQL 8.0.12, extended information is available for SELECT, DELETE, INSERT, REPLACE, and UPDATE statements. Prior to 8.0.12, extended information is available only for SELECT statements.
The Message value in SHOW WARNINGS output displays how the optimizer qualifies table and column names in the SELECT statement, what the SELECT looks like after the application of rewriting and optimization rules, and possibly other notes about the optimization process.

It is reasonable to add warnings about functions that can not be pushed to coprocessor when user executing an explain statement, so this pr add this information to make user easier to figure out why Selection or Aggregation is not pushed to coprocessor.

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

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

Side effects

Release note

• Add warning info for exprs that can not be pushed to storage layer for explain statement.

[ti-srebot]

/run-all-tests

[ti-srebot]

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

[hanfei1991]

LGTM

[ti-chi-bot]

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[fzhedu]

LGTM

[ti-chi-bot]

@fzhedu: Please use /LGTM instead of LGTM when you want to approve the pull request by comment.
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[windtalker]

/run-all-tests

[hanfei1991]

/LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• fzhedu
• hanfei1991

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[fzhedu]

/merge

[ti-chi-bot]

@fzhedu: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: 6d195e3

[windtalker]

/run-unit-test