Github user maryannxue commented on a diff in the pull request:
https://github.com/apache/spark/pull/19488#discussion_r144881001
--- 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 --
Then I think the solution we have right now (in my latest ci) is what we
need. But we need to refine the code comment there to further illustrate cases
like "first_value". And in general we may need better documentation regarding
the behavior of FIRST_VALUE and similar functions. Agree?
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