planner: fix wrong plan caused by shallow copy schema columns (#27798)

[ti-srebot]

cherry-pick #27798 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/27837

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

git push git@github.com:ti-srebot/tidb.git pr/27837:release-5.0-68e32bb44e37

---

What problem does this PR solve?

Issue Number: close #27797 close #26554

Problem Summary:
In agg push down, we make the schema of the Union and the first child of the Union(a DataSource in the case) share the same array.
Then in the column pruning, when we are pruning columns of the child of the Union, the schema of Union is also changed because they share the same slice.

What is changed and how it works?

Deep copy schema columns when push agg across Union.

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Reminiscent
• wshwsh12

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-srebot]

/run-all-tests

[ti-srebot]

@time-and-fate you're already a collaborator in bot's repo.

[wshwsh12]

Please resolve conflict

[time-and-fate]

/merge

[ti-chi-bot]

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

Commit hash: ee8492f

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@ti-srebot: Your PR was out of date, I have automatically updated it for you.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

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.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.