Fix subquery merging regression introduced in #11379 (#18260)

[arthurschreiber]

Description

This is a backport of vitessio#18260

Related Issue(s)

• Push down derived tables under route when possible vitessio/vitess#11379
• Fix subquery merging regression introduced in #11379 vitessio/vitess#18260

Checklist

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

Deployment Notes

[Copilot]

Pull Request Overview

Backports the fix for subquery merging regression introduced in issue vitessio#11379, as implemented in vitessio#18260.

• Updates expected planbuilder test cases to remove deprecated UncorrelatedSubquery instructions and adjust FieldQuery/Query fields
• Refines canMergeOnFilter to extract only column names using getColName before vindex lookup

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
go/vt/vtgate/planbuilder/testdata/select_cases.json	Adjusted JSON test expectations for subquery routes
go/vt/vtgate/planbuilder/operators/route_planning.go	Switched comparison.Left/Right to getColName calls in canMergeOnFilter

Comments suppressed due to low confidence (2)

go/vt/vtgate/planbuilder/operators/route_planning.go:395

• [nitpick] The variables left and right now hold *sqlparser.ColName values or nil. Consider renaming them to leftCol and rightCol for clarity.

left := getColName(comparison.Left)

go/vt/vtgate/planbuilder/operators/route_planning.go:395

• Add unit tests for canMergeOnFilter covering cases where comparison.Left or comparison.Right is not a simple column (e.g., aggregate expressions), to ensure getColName returning nil is handled correctly.

left := getColName(comparison.Left)