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]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.88%. Comparing base (32b8bd8) to head (96078c6).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19046      +/-   ##
==========================================
- Coverage   69.89%   69.88%   -0.01%     
==========================================
  Files        1612     1613       +1     
  Lines      215826   216021     +195     
==========================================
+ Hits       150857   150974     +117     
- Misses      64969    65047      +78     

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