vtgate: expand qualified stars without JOIN USING coalescing

[GrahamCampbell]

Description

The early rewriter applies JOIN USING coalescing to qualified stars. MySQL coalesces the join columns for an unqualified * only: tbl.* returns every column of the named table, join columns included, so select r.* from t l join t r using (col1) returns three columns on MySQL but two on Vitess, silently dropping r.col1 from the results. The right-hand side of the join loses its join columns; the left-hand side keeps them, so the wrong shape only shows on one side.

The fix clears the coalescing state when the star carries a qualifier, before the expander runs. Unqualified stars keep the existing coalesced expansion and ordering unchanged, pinned by the existing cases. Four new expansion cases cover both join sides, a mixed qualified pair, and a second table shape, and a planner golden pins the emitted SQL end to end; that file runs in the plan e2e suite, so CI compares the corrected expansion against MySQL directly.

This came out of reviewing #20633, whose declared-column-list validation turns the wrong expansion into a spurious VT03033 for select a from (select r.* from t l join t r using (col1)) x(a, b, c). The validation change now stacks on this fix. Since the wrong column set is returned to clients on current releases, this is probably worth backporting.

Related Issue(s)

Found while addressing review feedback on #20633.

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

Qualified stars in queries with JOIN USING now expand to every column of the named table, matching MySQL: previously the join columns were omitted from the right-hand table's tbl.* expansion, and results silently differed from MySQL. 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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