vtgate: fix union merging through reference-table alternates

[GrahamCampbell]

Description

A UNION DISTINCT between a reference table and a table in the keyspace holding one of its copies panics with VT13001: [BUG] did not expect this method to be called. When the two sides' keyspaces differ, the planner makes them mergeable by swapping in the alternate route prebuilt at route-creation time, but unions merge later, during horizon planning, and the prebuilt route still holds the unplanned bare table it was created with. As soon as the union pushes its result columns into that source — the weight-string columns DISTINCT needs — the bare table operator panics in Table.AddColumn. UNION ALL survives because it pushes nothing.

The fix stops using the prebuilt alternate as the merge payload. A declined cross-keyspace pairing is retried by resolving every real table under the moved side to its copy in the other side's keyspace through the VSchema — the table itself, a ReferencedBy copy, or a reference source, so routing rules stay in effect — and pointing the planned operators at those copies in place, undoing the move when the retry still cannot merge. Predicates and projections pushed after route creation are preserved because the operators are kept, physical renames keep the original name as an alias so column references still resolve, and later column pushes land on operators the planner has analyzed. A side already composed of a merged union is not moved: the remaining phases no longer normalize the shape that produces, so composite pairings stay split for now. Join and subquery merging keep the prebuilt-route mechanism: they merge during physical planning, when every route source is still a bare table, and that path is covered and correct.

Nine planner cases in reference_cases.json cover both merge orders, the differing-physical-name rewrite, predicate preservation through the rewrite, the composite pairings that stay split, a no-copy control, and pin the already-working UNION ALL path; the four single-table DISTINCT cases fail on main with the panic. The reference-copy cases are skip_e2e like the rest of that file's, since the e2e fixture does not define the copy tables. Recursive CTE merging shares the swap through prepareInputRoutes and likely has the same latent panic, left for a follow-up. This is part one of the fix for #20754, and since the panic exists in released versions it is probably worth backporting.

Related Issue(s)

Part one of #20754.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

UNION DISTINCT queries pairing a reference table with a table in a keyspace holding one of its copies previously failed to plan with an internal error; they now plan as a single merged route. No migrations or configuration changes.

AI Disclosure

This PR was written primarily by Fable.

---

graph LR
    subgraph json [JSON support]
        direction TB
        PR20625["#20625 (merged)<br>mysql/json: fix MarshalTo discarding accumulated output for nested blob and bit values"]
        PR20632["#20632<br>vtgate: preserve IN value lists in complex aggregate projections"]
        PRA["#20691<br>evalengine: disable the static IN hash table for JSON operands"]
        PR20682["#20682 (draft)<br>evalengine: support constant-folded JSON values as literals"]
        PR20683["#20683 (draft)<br>evalengine, sqlparser: MySQL comparison domains; nested BETWEEN parentheses"]
        PR20626["#20626 (draft)<br>vtgate: support cross-shard JSON_ARRAYAGG and JSON_OBJECTAGG"]
        PR20625 --> PR20682
        PRA --> PR20682
        PR20682 --> PR20683
        PR20632 --> PR20626
        PR20683 --> PR20626
    end

    subgraph union [Union routing]
        direction TB
        PR20628["#20628<br>vtgate: track per-source copies of pushed join predicates so merges skip them"]
        PR20629["#20629<br>vtgate: fix None routing handling when merging unions"]
        PR20630["#20630 (draft)<br>vtgate: merge unions on join-predicate-free routings when all sources agree"]
        PR20755["#20755<br>vtgate: fix union merging through reference-table alternates"]
        PR20756["#20756 (draft)<br>vtgate: merge empty reference branches through rewritten copies"]
        PR20628 --> PR20630
        PR20629 --> PR20630
        PR20630 --> PR20756
        PR20755 --> PR20756
    end

    subgraph cte [Recursive CTEs]
        direction TB
        PR20759["#20759<br>vtgate: expand qualified stars without JOIN USING coalescing"]
        PR20633["#20633 (draft)<br>vtgate: validate the declared column list length of CTEs and derived tables"]
        PR20631["#20631 (draft)<br>vtgate: bind recursive CTE column filters as arguments in unmerged term queries"]
        PR20759 --> PR20633
        PR20633 --> PR20631
    end

    json ~~~ union ~~~ cte

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