Fix column offset tracking for UNIONs to be case insensitive.

[arthurschreiber]

Description

This is a followup to #19046.

It fixes an issue where UNION queries couldn't push down predicates correctly when a column name in the first SELECT of the union was using upper case characters while the predicate used lower case names. This caused an additional FILTER operation to be added to queries, which in turn could prevent route merging.

This pull request fixes this issue by ensuring that both building the offset map and lookups into the offset map use lowercase column names.

This also fixes an issue introduced via #19046, where we added the join predicate unwrapping. This seemingly fixed the issue that pull request attempted to fix, but the fix broke down with the changes added in this PR - it caused all sorts of issues with information_schema queries.

Instead of unwrapping the JoinPredicate, we create new JoinPredicate objects for every source of the UNION. This allows the existing code to modify/restore the JoinPredicates pushed down into the UNIONs correctly in cases where those modifications need to be reverted (which can happen if LHS and RHS of an ApplyJoin get merged).

Related Issue(s)

Related to #19113

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

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[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.91%. Comparing base (d38ee62) to head (44e634c).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/planbuilder/operators/union.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #19139   +/-   ##
=======================================
  Coverage   69.90%   69.91%           
=======================================
  Files        1613     1613           
  Lines      216076   216071    -5     
=======================================
+ Hits       151055   151060    +5     
+ Misses      65021    65011   -10     

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[mattlord]

I'm not saying this is wrong, I don't know so asking. Is panic (crashing the vtgate) the right thing to do vs returning a query error?

[arthurschreiber]

This is a bit confusing, but panic inside the planner package does not crash the vtgate, it's the canonical way to return an error from inside the planner back to the vtgate.