houqp commented on a change in pull request #619:
URL: https://github.com/apache/arrow-datafusion/pull/619#discussion_r658910893
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File path: datafusion/src/optimizer/projection_push_down.rs
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@@ -75,9 +78,12 @@ fn get_projected_schema(
//
// we discard non-existing columns because some column names are not part
of the schema,
// e.g. when the column derives from an aggregation
- let mut projection: Vec<usize> = required_columns
+ //
+ // Use BTreeSet to remove potential duplicates (e.g. union) as
+ // well as to sort the projection to ensure deterministic behavior
+ let mut projection: BTreeSet<usize> = required_columns
Review comment:
curious when should we use a HashSet v.s. BTreeSet?
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