alamb commented on a change in pull request #1618:
URL: https://github.com/apache/arrow-datafusion/pull/1618#discussion_r789943797
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File path: datafusion/src/optimizer/filter_push_down.rs
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@@ -253,33 +176,107 @@ fn split_members<'a>(predicate: &'a Expr, predicates:
&mut Vec<&'a Expr>) {
}
}
+// For a given JOIN logical plan, determine whether each side of the join is
preserved.
+// We say a join side is preserved if the join returns all or a subset of the
rows from
+// the relevant side - i.e. the side of the join cannot provide nulls. Returns
a tuple
+// of booleans - (left_preserved, right_preserved).
+fn lr_is_preserved(plan: &LogicalPlan) -> (bool, bool) {
+ match plan {
+ LogicalPlan::Join(Join { join_type, .. }) => match join_type {
+ JoinType::Inner => (true, true),
+ JoinType::Left => (true, false),
+ JoinType::Right => (false, true),
+ JoinType::Full => (false, false),
Review comment:
`Full` is `FULL OUTER` join... I think in that case the inputs are both
preserved (as in you can't push filters down through the join if it isn't for
that column)
Though that doesn't seem consistent with `JoinType::Inner`
Maybe
```rust
JoinType::Inner => (false, false),
```
as predicates *can* be pushed down both sides there 🤔
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