Fix cross shard/keyspace joins with derived tables containing a UNION.

[arthurschreiber]

Description

This fixes an issue in the planner where a cross shard/keyspace join with a derived table wrapping a non-mergeable UNION would fail with VT13001: [BUG] could not find the column '...' on the UNION.

I'm not sure if this should be backported. Probably it should.

If I understand the flow of this code correctly, when sqlparser.CopyOnRewrite is called with a JoinPredicate, JoinPredicate.Clone is called which replaces the JoinPredicate in the JoinPredicates tracker with the modified instance (that contains the rewritten expr). Later in the flow that rewritten join predicate is used but at that point the column references are incorrect.

To "workaround" this, we're now unwrapping JoinPredicates into their underlying expressions, so the information in the tracker does not get modified when the rewrite happens through sqlparser.CopyOnRewrite.

Related Issue(s)

Fixes: #19113

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

AI Disclosure

Claude code was used to find the cause of the error and to come up with a fix. The fix looks correct to me and solves the particular issue.

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

❌ Patch coverage is 76.19048% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.89%. Comparing base (749fe54) to head (6cffd48).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/planbuilder/operators/union.go	76.19%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19046      +/-   ##
==========================================
+ Coverage   69.88%   69.89%   +0.01%     
==========================================
  Files        1610     1610              
  Lines      215431   215523      +92     
==========================================
+ Hits       150551   150647      +96     
+ Misses      64880    64876       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.