planner: fix incorrect result of set type for merge join (#25672)

[ti-srebot]

cherry-pick #25672 to release-5.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25695

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

git push git@github.com:ti-srebot/tidb.git pr/25695:release-5.1-574de5e5ec00

---

What problem does this PR solve?

Issue Number: close #25669

Problem Summary:

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:
Avoid merge join using set index.

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)
• No code

Side effects

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

Release note

• Fix incorrect result of set type for merge join

[ti-srebot]

/run-all-tests

[ti-srebot]

@wshwsh12 you're already a collaborator in bot's repo.

[qw4990]

PTAL @XuHuaiyu

[XuHuaiyu]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• XuHuaiyu
• qw4990

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[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: 3fb3229

[ti-chi-bot]

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