Improve UNION query merging

[arthurschreiber]

Description

This PR adds two "naive" improvements to allow UNION queries to be merged in more cases.

The first change allows UNION subqueries that use IN conditions to be merged together correctly. It only works if tuples on the two subqueries we try to merge match exactly (same elements in the exact same order), or the tuple was normalized to the same bind variable (see the change below). This could be improved upon by doing a true "set" comparison (so that the order of values and duplicates don't affect the merge result).

This probably should also be extended to allow merging queries where one side uses foobar IN (1) and the other side uses foobar = 1. Merging foobar IN (1) with foobar = 1 already works, so I just added a test case to make sure it's covered.

This change is currently missing test coverage, but I'll make sure to add some.
Various test cases to cover this change have been added.

The second change improves bind variable normalization so that bind variables for ValTuple get de-duplicated, similar to other bind variables. That de-duplication only happens if the tuples contain exactly the same values (with matching types) in the same order.

This could probably be improved upon by pre-sorting and de-duplicating the tuple values, to ensure that tuples like (1, 2, 3) (1, 1, 2, 3) and (3, 2, 1) all end up with the same bind variable value (and thus allow UNION queries that use such tuples for routing to be merged).

Related Issue(s)

#18288

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
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

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[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.47%. Comparing base (fcfa0c8) to head (5b6a8c2).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/sqlparser/normalizer.go	84.61%	2 Missing ⚠️
.../vt/vtgate/planbuilder/operators/route_planning.go	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18289      +/-   ##
==========================================
- Coverage   67.48%   67.47%   -0.02%     
==========================================
  Files        1603     1603              
  Lines      262337   262384      +47     
==========================================
+ Hits       177034   177039       +5     
- Misses      85303    85345      +42     

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[arthurschreiber]

This is a bit awkward - We can't use bval directly as the map key, because Values on querypb.BindVariable is a slice, and structs containing slices can't be used as map keys.

So I'm using MarshalVT in combination with string to generate a value that can be used as a key. This is probably not super-optimal, but 🤷