expression: push down left/right/char_length (#24840)

[ti-srebot]

cherry-pick #24840 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/25563

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

git push git@github.com:ti-srebot/tidb.git pr/25563:release-5.0-087f392d22d7

---

What problem does this PR solve?

Issue Number: close pingcap/tiflash#1943

Problem Summary: push down left/right without cosidering collation, char_length aims to verification.

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:

Related changes

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

Check List

Tests

• 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

• push down left/right/char_length to tiflash

[ti-chi-bot]

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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[XuHuaiyu]

New feature, do not need to cherrypick