planner: enforce projection push down (#25450)

[ti-srebot]

cherry-pick #25450 to release-5.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25741

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git push git@github.com:ti-srebot/tidb.git pr/25741:release-5.1-9ca449b8bd3a

---

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary: if enforced MPP, we push down projection to tiflash.

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Integration test

Release note

• No release note

[ti-srebot]

/run-all-tests

[ti-srebot]

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

[LittleFall]

LGTM

[ti-chi-bot]

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[fzhedu]

/LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• fzhedu
• lzmhhh123

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[fzhedu]

/merge

[ti-chi-bot]

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

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Commit hash: a40b953

[fzhedu]

/run-all-tests

[fzhedu]

/merge

[ti-chi-bot]

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

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Commit hash: dfd7c71

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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