Fix information schema queries not being merged correctly

[arthurschreiber]

Description

Various queries against information_schema tables were not being merged correctly.

This pull request contains multiple interlinked fixes:

Column offset map for UNION queries

The column offset map used for UNION queries was doing column name comparisons incorrectly. The offset map was built using the user provided column names, but lowercased column names were used during lookups into the map. Column names in MySQL are case insensitive, so we now lowercase both the values used for creating the map as well as looking up the offsets.

This affects not just information_schema UNION queries, but was noticed there because the schema for information_schema tables that ships with Vitess uses upper cased column names.

Add support for merging UNION queries targeting information_schema tables

JOIN operations against information_schema tables supported being merged if the LHS and RHS targeted the same data sources (e.g. if the same schemas or tables were queried). This logic was missing from the UNION merging implementation.

Fix information_schema merge ability rules (affects both JOIN and UNION merging)

Merging of information_schema routes was only possible if the table schema list of the LHS and the RHS of the merge matched fully (exact match of all values). This was artificially limiting the routes that could be merged, as the actual requirement is that the table schema lists only need to intersect (share one common value).

---

This also updated the planner debug output for DBA routes (used for information_schema queries) to include the SysTableTableSchema and SysTableTableName information to make it a bit easier to understand what's going on.

Related Issue(s)

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 66.66667% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.88%. Comparing base (93e4601) to head (991dea5).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...gate/planbuilder/operators/info_schema_planning.go	18.75%	13 Missing ⚠️
go/vt/vtgate/planbuilder/operators/union.go	80.00%	1 Missing ⚠️
...o/vt/vtgate/planbuilder/operators/union_merging.go	95.83%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #19138   +/-   ##
=======================================
  Coverage   69.88%   69.88%           
=======================================
  Files        1613     1613           
  Lines      216019   216055   +36     
=======================================
+ Hits       150960   150999   +39     
+ Misses      65059    65056    -3     

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[arthurschreiber]

#19139 contains a better approach / fix.