branch-4.0: [fix](nereids) clamp the merged limit of MERGE_TOP_N by the parent offset #64306#64352
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branch-4.0: [fix](nereids) clamp the merged limit of MERGE_TOP_N by the parent offset #64306#64352github-actions[bot] wants to merge 1 commit into
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…fset (#64306) `MergeTopNs` (the `MERGE_TOP_N` rewrite rule) merges a parent `TopN` into its child `TopN` when their order keys are compatible. When the parent `TopN` carries a non-zero `OFFSET`, the merged limit was computed as `min(parent.limit, child.limit)`, which ignores that the parent offset consumes rows from the child's output. The merged `TopN` therefore keeps too many rows and the query returns a wrong result. Example: ```sql SELECT * FROM (SELECT k, v FROM t ORDER BY k LIMIT 5) s ORDER BY k LIMIT 3 OFFSET 4; ``` The inner `ORDER BY k LIMIT 5` yields 5 rows; the outer `LIMIT 3 OFFSET 4` skips 4 of them, so only 1 row should remain. Before this PR the rule merged the two `TopN` into `OFFSET 4 LIMIT 3` (instead of `OFFSET 4 LIMIT 1`), so it returned 3 rows. Fix: clamp the merged limit by `max(child.limit - parent.offset, 0)`, the same semantics already used by `MergeLimits.mergeLimit` for consecutive limits. The bug only triggers when the outer `TopN` has a non-zero offset (offset = 0 makes both formulas equal). The existing unit test `MergeTopNsTest.testOffset` asserted the buggy value (`limit == 10`, while the correct value is `9`); this PR corrects that assertion as well. ### Release note Fix the wrong result produced by the `MERGE_TOP_N` optimization when an outer `ORDER BY ... LIMIT` carries a non-zero `OFFSET` over an inner `ORDER BY ... LIMIT`.
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Cherry-picked from #64306