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.

Related Issue(s)

N/A

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.