neilconway commented on code in PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#discussion_r3603579321
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datafusion/optimizer/src/eliminate_join.rs:
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@@ -394,6 +406,30 @@ fn rewrite_join(
let (visible_left, visible_right) = split_join_output_columns(&join, live);
+ // A LEFT JOIN preserves every left row, so when nothing above the join
+ // references the right side's columns and the right side cannot multiply
+ // left rows (the ancestors are duplicate-insensitive, or the right side
+ // is unique on the join keys), the join has no observable effect and can
+ // be replaced by its left input. A join filter cannot prevent this: it
+ // only decides whether a left row is matched or null-padded, and either
+ // way the row is emitted.
+ if join.join_type == JoinType::Left
Review Comment:
I'm not sure there's a rush to land left-join support super fast 😊 But sure,
if you'd prefer to split this into two PRs, that's fine with me.
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