Github user cloud-fan commented on a diff in the pull request: https://github.com/apache/spark/pull/19488#discussion_r144684245 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/planning/patterns.scala --- @@ -205,14 +205,17 @@ object PhysicalAggregation { case logical.Aggregate(groupingExpressions, resultExpressions, child) => // A single aggregate expression might appear multiple times in resultExpressions. // In order to avoid evaluating an individual aggregate function multiple times, we'll - // build a set of the distinct aggregate expressions and build a function which can + // build a map of the distinct aggregate expressions and build a function which can // be used to re-write expressions so that they reference the single copy of the - // aggregate function which actually gets computed. - val aggregateExpressions = resultExpressions.flatMap { expr => + // aggregate function which actually gets computed. Note that aggregate expressions + // should always be deterministic, so we can use its canonicalized expression as its --- End diff -- good point, I think `first` is a nondeterministic function, SQL Server also document it: https://docs.microsoft.com/en-us/sql/relational-databases/user-defined-functions/deterministic-and-nondeterministic-functions (search for FIRST_VALUE). But here, although `first` is nondeterministic, I think `select first(a) - first(a) from ...` should return 0, as its result should be consistent in-query. CC @gatorsmile @hvanhovell
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