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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