expression: fix different results for greatest when vectorized is off (#29438)

[ti-srebot]

cherry-pick #29438 to release-4.0
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/29916

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

git push [email protected]:ti-srebot/tidb.git pr/29916:release-4.0-8ae81b38525b

---

Signed-off-by: guo-shaoge [email protected]

What problem does this PR solve?

Issue Number: close #29434

Problem Summary:
The correct semantics of greatest/least(col_datetime, col_string):

1. Convert all args as String
2. EvalString to get arg value
3. Parse string as datetime. If it's a valid datetime, use string(datetime) to compare. Otherwise use the result of EvalString to compare.

What is changed and how it works?

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

expression: fix different results for greatest when vectorized is off.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• XuHuaiyu
• wshwsh12

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[ti-srebot]

/run-all-tests

[ti-srebot]

@guo-shaoge you're already a collaborator in bot's repo.

[XuHuaiyu]

LGTM

[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: 45ab2e4

[VelocityLight]

/merge