planner: arm engine-restricted alternative logical plan rounds from round 1's engine usage (WIP)

[terry1purcell]

This is an alternative design to #70005; only one of the two will ship.

The physical search picks a storage engine per DataSource through local cost comparisons, so per-table index access can win table by table even when a homogeneous whole-statement plan (in particular an MPP plan) would win overall — and that homogeneous alternative is never costed as a unit. #70005 arms the tikv-only / tiflash-only rebuild rounds only when round 1's chosen plan already mixes both engines. This PR instead arms each round from the engines round 1 actually read, so an all-TiKV plan still gets a whole-statement TiFlash/MPP alternative costed against it — the case #70005 structurally cannot reach.

InspectPlanShape walks round 1's chosen plan once for its engine usage and for whether it joins or aggregates, descending into reader subtrees because pushed-down aggregations and MPP joins live inside cop and MPP tasks. The tikv-only round arms when round 1 read from TiFlash anywhere; the tiflash-only round arms when it read from TiKV anywhere, including the all-TiKV case. Shared gates keep the extra rounds off statements that cannot benefit: a join or aggregation must be present (keeps the rounds off the OLTP fast path), no READ_FROM_STORAGE hint may pin an engine, and enforced MPP is excluded (its cost discount would distort the cross-round comparison). The tiflash-only round additionally requires every table to have a TiFlash access path and MPP to be allowed.

What problem does this PR solve?

Issue Number: ref #70021

Problem Summary:

The alternative logical plan driver only builds a fully engine-homogeneous plan when round 1 already mixed engines. An all-TiKV plan can still hide a cheaper whole-statement MPP plan that was never costed, and that case is not covered.

What changed and how does it work?

• New physicalop.InspectPlanShape returns, from a single walk of round 1's chosen plan, which storage engines it read from and whether it contains a join or aggregation. It descends into reader subtrees (TablePlan / IndexPlan / partial plans) because pushed-down aggregations and MPP joins live inside cop and MPP tasks.
• optimize records those facts as StmtCtx signals after round 1 and pre-computes round eligibility from them (so later rounds' side-effect signals cannot leak into arming).
• Two rounds, tikv-only and tiflash-only, rebuild the plan with IsolationReadEngines restricted to a single storage engine; the existing strict-< cross-round cost comparison picks the winner. Arming and gates are described above.
• Per-round setup now returns its own cleanup closure instead of using package-level saved state, so concurrent optimize calls in different sessions cannot clobber one another's saved values.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

TestAlternativeEngineRoundArming in pkg/planner/core/casetest/mpp/engine_rounds_test.go covers each arming and gating case, including the headline all-TiKV-plan-arms-tiflash-only case, the missing-replica skip, the join/agg gate, the hint gate, enforced MPP, and the feature-off case.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

🤖 Generated with Claude Code

https://claude.ai/code/session_01TvP1rNMDfN8B6S7jx14Y7M

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