neilconway commented on code in PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#discussion_r3596758392
##########
datafusion/optimizer/src/eliminate_join.rs:
##########
@@ -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
+ && visible_right.is_empty()
+ && (duplicate_insensitive
+ || side_unique_on_join(
+ join.right.schema(),
+ join.on.iter().map(|(_, right)| right),
+ join.null_equality,
+ ))
Review Comment:
Consider refactoring this into a helper, so we can use it here and also in
`rewritten_join_type`?
##########
datafusion/optimizer/src/eliminate_join.rs:
##########
@@ -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:
What about supporting `RIGHT JOIN`s as well?
##########
datafusion/sqllogictest/test_files/joins.slt:
##########
@@ -5608,3 +5608,316 @@ set datafusion.execution.target_partitions = 4;
statement ok
reset datafusion.execution.batch_size;
+
+##########
+# Eliminate unused LEFT JOINs (`EliminateJoin` rule)
Review Comment:
Can we add test cases for `LIMIT` and `LEFT JOIN LATERAL`?
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