zhidongqu-db opened a new pull request, #56052: URL: https://github.com/apache/spark/pull/56052
…e synthetic join ### What changes were proposed in this pull request? This PR changes `RewriteNearestByJoin` to construct its synthetic cross-join with the user's `joinType` (`Inner` or `LeftOuter`) instead of always using `LeftOuter`. The `Generate` operator's `outer` flag continues to be derived from `joinType == LeftOuter`, so the externally observable semantics are unchanged. ### Why are the changes needed? The original implementation hardcoded the synthetic join to `LeftOuter` and justified it on the grounds that `LEFT OUTER` and `INNER` are equivalent for an unconditioned join when the right side is non-empty, and `Generate(outer = false)` would drop unwanted rows for `INNER` when right is empty. That reasoning holds for correctness but has a major performance cost: - **`INNER NEAREST BY` cannot be planned as a Cartesian product.** Spark's strategy picks `CartesianProductExec` only for `Inner` joins with no condition; an unconditioned `LeftOuter` join falls back to `BroadcastNestedLoopJoin`, which tries to broadcast the right side. When the right relation is large, the broadcast either OOMs or exceeds `spark.sql.autoBroadcastJoinThreshold` and the planner is left with no good option. `CartesianProductExec` partitions both sides and streams pairs, so it scales naturally with right-side size. Respecting the user's `INNER` join type re-enables this strategy for the common `INNER NEAREST BY` case. - It also makes the EXPLAIN output misleading (shows `LeftOuter` even though the user wrote `INNER`). - For `INNER` with an empty right side, the old plan generates one row per left input and then filters them away via `Generate(outer = false)` and the `size(matches) > 0` filter -- extra work that respecting `joinType` avoids at the source. ### Does this PR introduce _any_ user-facing change? No change in query results. `EXPLAIN` output for `INNER NEAREST BY` queries now shows `Inner` rather than `LeftOuter` for the synthetic join node, and the physical plan for such queries can now use `CartesianProductExec` instead of `BroadcastNestedLoopJoin` when the right relation is too large to broadcast. ### How was this patch tested? - `RewriteNearestByJoinSuite`: `expectedRewrite` now takes a `joinType: JoinType` and the existing tests (similarity/distance x inner/leftouter, EXACT, boundary k, self-join, nondeterministic ranking) assert the synthetic join matches the user's join type. - Golden file `sql-tests/results/join-nearest-by.sql.out` ### Was this patch authored or co-authored using generative AI tooling? Coauthored-by: Claude Code (Opus 4.7), human-reviewed and tested Closes #56023 from zhidongqu-db/respect-nn-join-type. Authored-by: Zero Qu <[email protected]> <!-- Thanks for sending a pull request! Here are some tips for you: 1. 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