server: use stmt should apply to subsequent stmts in multi-stmt mode (#26905)

[ti-srebot]

cherry-pick #26905 to release-5.1
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/27397

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

git push [email protected]:ti-srebot/tidb.git pr/27397:release-5.1-20797003212d

---

What problem does this PR solve?

Issue Number: close #26688

Problem Summary:

When multi-statement is enabled, chaining statements following a "use" stmt was not successful. i.e.

use test; create table if not exists t1 (id int); drop table t1;

This is because of prefetching for point plans. It seems unsafe to prefetch the plan if the schema could be changed (but the query might still be valid). So my solution was to disable prefetching if there is a use stmt.

What is changed and how it works?

What's Changed:

Fixes a bug in multi-stmt mode.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU (Point plan cache will be disabled in some edge-case scenarios that are not expected to appear in most user queries).
• 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

When executing multiple statements in one `COM_QUERY` API call, TiDB did not correctly handle the case that a `use db` statement applies to all remaining statements.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• xhebox

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[morgo]

I don't think this is a common issue, so I am going to close this for now. If we receive additional feedback we can consider cherry picking.