adriangb commented on code in PR #19538:
URL: https://github.com/apache/datafusion/pull/19538#discussion_r2721546456
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datafusion/physical-expr/src/projection.rs:
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@@ -806,17 +807,65 @@ pub fn update_expr(
RewrittenInvalid,
}
+ // Track Arc pointers of columns created by pass 1.
+ // These should not be modified by pass 2.
+ // We use Arc pointer addresses (not name/index) to distinguish
pass-1-created columns
+ // from original columns that happen to have the same name and index.
+ let mut pass1_created: HashSet<usize> = HashSet::new();
+
+ // First pass: try to rewrite the expression in terms of the projected
expressions.
+ // For example, if the expression is `a + b > 5` and the projection is `a
+ b AS sum_ab`,
+ // we can rewrite the expression to `sum_ab > 5` directly.
+ //
+ // This optimization only applies when sync_with_child=false, meaning we
want the
+ // expression to use OUTPUT references (e.g., when pushing projection down
and the
+ // expression will be above the projection). Pass 1 creates OUTPUT column
references.
+ //
+ // When sync_with_child=true, we want INPUT references (expanding OUTPUT
to INPUT),
Review Comment:
Yeah this method has always... confused me. At the very least the boolean
argument is not great (I understand there's a lot of shared code between the
branches and that's why it is this way).
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