[release-23.0] vtgate: fix None routing handling when merging unions (#20629)

[vitess-bot]

Description

This is a backport of #20629

[vitess-bot]

Hello @GrahamCampbell, there are conflicts in this backport.

Please address them in order to merge this Pull Request. You can execute the snippet below to reset your branch and resolve the conflict manually.

Make sure you replace origin by the name of the vitessio/vitess remote

git fetch --all
gh pr checkout 20940
git reset --hard origin/release-23.0
git cherry-pick -m 1 39bcd63d47db82a19fb998cfedfa766c7d841769

[Copilot]

Pull request overview

Backports corrected NoneRouting handling when merging vtgate unions, preventing dropped rows and invalid cross-keyspace plans.

Changes:

• Prioritizes NoneRouting pairings and tracks inferred keyspaces.
• Adds table/keyspace validation for safe union merges.
• Adds planner and end-to-end regression coverage.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
union_cases.json	Adds union planning fixtures.
dml_cases.json	Adds targeted DML planning fixtures.
union_merging.go	Implements safe None-route merging.
route.go	Preserves inferred None routing state.
route_planning.go	Adds keyspace accounting helpers.
route_planning_test.go	Tests routing and keyspace accounting.
misc_routing.go	Adds inferred-keyspace metadata.
union_test.go	Adds execution regressions.

Suppressed comments (2)

go/vt/vtgate/planbuilder/operators/route_planning_test.go:310

• This second vindexes.Source literal also tries to initialize the promoted Name field directly, which is not permitted for keyed literals on the release-23 type and causes a compile error. Initialize its embedded TableName.

					Source: &vindexes.Source{
						Name: sqlparser.NewIdentifierCS("src"),
					},

go/test/endtoend/vtgate/queries/union/union_test.go:94

• These assertions cover behavior introduced by this release-23 backport, but the query suite also runs against older vtgate binaries and gates version-specific cases elsewhere in this file. Put these two assertions behind BinaryIsAtLeastAtVersion(23, "vtgate") so compatibility runs do not fail on binaries that predate the fix.

			mcmp.AssertMatches("select 1 from dual union select id1 from t1 where id1 in (null)", "[[INT64(1)]]")
			mcmp.AssertMatches("select id1 from t1 where id1 in (null) union select 1 from dual", "[[INT64(1)]]")

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