planner/core: remove the union branch with dual table. (#25218)

[hanfei1991]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

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

• 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

• planner/core: remove the union branch with dual table.

[fzhedu]

/LGTM

[ti-chi-bot]

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[ti-chi-bot]

[REVIEW NOTIFICATION]

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• fzhedu
• qw4990

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[hanfei1991]

/merge

[ti-chi-bot]

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

Commit hash: da403ff

[hanfei1991]

/merge

[ti-chi-bot]

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

Commit hash: 6290ef1

[ti-chi-bot]

@hanfei1991: Your PR was out of date, I have automatically updated it for you.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

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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