Braedon-Wooding-Displayr opened a new pull request, #24428:
URL: https://github.com/apache/datafusion/pull/24428
ChildFilterDescription::from_child (and from_child_with_allowed_indices)
built a FilterRemapper before checking whether parent_filters was empty.
FilterRemapper::new indexes every column of the child's schema into a HashMap,
and with no filters to remap that index is never used: remap_filters returns an
empty description regardless of what the remapper contains.
Each plan node produces one ChildFilterDescription per child, so a union
pays this cost once per child. On a 250,000-column scan a sampling profile put
FilterRemapper::new at 794 of 20,373 samples, almost entirely HashMap insertion
and hashing. Return ChildFilterDescription::empty() directly when
parent_filters is empty, before the remapper is built.
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