kumarUjjawal commented on PR #21621:
URL: https://github.com/apache/datafusion/pull/21621#issuecomment-4458768522
@adriangb Wanted your opinion on something, now that we deliberately emit
`Sort(exprs, fetch=N) → Join → Sort(exprs, fetch=N)`, the outer Sort becomes
redundant whenever the join is provably 1-to-≤1 on the preserved key (e.g.
unique constraint on the other side, or upstream DISTINCT/GROUP BY on the join
column). In that case the pushed Sort's ordering survives the join and the
outer Sort + LIMIT could collapse to a plain Limit(N).
Few questions:
1. Does the FD info on Join's output schema already carry enough to detect
this, or would it need more wiring?
2. Better as a new rule (EliminateRedundantOuterSort) or an extension to
an existing one (EliminateLimit / a Sort-dedup pass)?
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