vtgate: route inserts on pinned sharded tables to their pinned shard

[officialasishkumar]

Description

This PR fixes inserts for pinned tables in sharded keyspaces.

Pinned tables already carry a fixed keyspace id in the vschema, and generic sharded routing already treats them as single-shard EqualUnique routes. The insert pipeline was bypassing that routing path and unconditionally requiring primary vindex values, which caused INSERT into pinned tables to fail with VT09001.

This change carries the pinned destination through insert planning and execution, then routes inserts directly to the pinned shard when there are no insert vindex columns to evaluate. Regression coverage was added in both the engine and vtgate executor tests.

Related Issue(s)

Fixes #19529

Checklist

• "Backport to:" labels not necessary, not applied
• Not backported, so no backport justification provided
• Tests were added
• Did the new or modified tests pass consistently locally and on CI?
• Documentation is not required

Deployment Notes

None.

AI Disclosure

This PR was written with assistance from OpenAI Codex. I reviewed the final diff and local test results.

[vitess-bot]

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[Copilot]

Pull request overview

This PR fixes INSERT routing for pinned tables in sharded keyspaces by propagating the pinned keyspace-id destination into insert planning/execution so inserts can be sent directly to the pinned shard when no vindex evaluation is needed.

Changes:

• Carry a pinned-table TargetDestination through insert planning to avoid requiring a primary vindex for pinned tables with no vindexes.
• Add execution support for single-destination inserts (resolve destination → execute on exactly one shard).
• Add regression tests in both vtgate executor and engine layers.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go/vt/vtgate/planbuilder/operator_transformers.go	Sets InsertCommon.TargetDestination for pinned tables with no column vindexes during insert plan construction.
go/vt/vtgate/engine/insert_common.go	Adds TargetDestination to InsertCommon and a helper to execute inserts by an explicit shard destination.
go/vt/vtgate/engine/insert.go	Routes sharded inserts directly by TargetDestination when present.
go/vt/vtgate/engine/insert_select.go	Adds a TargetDestination fast-path for sharded INSERT ... SELECT and includes destination in primitive description.
go/vt/vtgate/engine/insert_test.go	Adds an engine-level regression test for sharded pinned inserts.
go/vt/vtgate/executor_dml_test.go	Adds an executor-level regression test ensuring pinned-table inserts only hit the pinned shard.

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[chatgpt-codex-connector]

Propagate pinned destination to INSERT ... SELECT plans

This change wires TargetDestination only in buildInsertPrimitive, which fixes INSERT ... VALUES but leaves the INSERT ... SELECT transformer path (transformInsertionSelection) constructing engine.InsertSelect without a pinned destination. For pinned sharded tables that have no insert vindexes, InsertSelect will still fall through to sharded vindex routing and call processVindexes with an empty ColVindexes slice, so INSERT ... SELECT on pinned tables remains broken instead of being routed to the pinned shard.

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[officialasishkumar]

Wired TargetDestination for pinned tables in the INSERT...SELECT path too, matching the existing logic in buildInsertPrimitive (commit f322ddb).