viirya commented on a change in pull request #27224: [SPARK-30523][SQL] -
Collapse nested aggregates
URL: https://github.com/apache/spark/pull/27224#discussion_r367188262
##########
File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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@@ -964,6 +965,155 @@ object CombineFilters extends Rule[LogicalPlan] with
PredicateHelper {
}
}
+/**
+ * Combines two adjacent [[Aggregate]] operators into one, if the first one is
not necessary.
+ *
+ * If we are referencing the outputs of aggregate functions in the inner
aggregate from the outer
+ * one, check if they are being used in outer aggregates in a way that can be
collapsed into a
+ * single aggregate. A sum of sums, or a max of max, or min of min are all
collapsible.
+ * avg over avg will not be collapsible because different number of raw rows
will have contributed
+ * to the partial averages of the inner aggregate
+ *
+ * Min an Max can be folded in the case described above, or if they are
referencing
+ * the group by columns, as they can safely be computed just using the set of
+ * unique values.
+ */
+object CombineAggregates extends Rule[LogicalPlan] with PredicateHelper {
+
+ /**
+ * The aggregate expression list includes both aggregate expressions and
+ * the projected group by keys, this filters out the aggregate expressions
+ * in the list leaving just the group by keys. It also unwraps aliases to
+ * just give a list of the projected grouping expressions themselves.
+ */
+ def justProjectedGroupExprs(aggExprs: Seq[NamedExpression],
+ groupExprs: Seq[Expression]): Seq[NamedExpression] = {
+ aggExprs.filter(namedEx =>
+ groupExprs.exists(_.semanticEquals(unwrapAlias(namedEx)))
+ )
+ }
+
+ def unwrapAlias(ex: Expression): Expression = {
+ if (ex.isInstanceOf[Alias]) ex.children.head
+ else ex
+ }
+
+ /**
+ * Pulls up references to aliases from an earlier operator and replaces them
with the
+ * raw expression they are associated with.
+ *
+ * The output name of the original expression is assumed to be the desired
final name
+ * of the rewritten expression, so if necessary an alias is added to ensure
the output
+ * name is correct.
+ *
+ * @param ex expression to re-write
+ * @param aliasMap aliases from the input operator, mapped to their
expressions
+ * @return rewritten expression with intermediate aliases removed
+ */
+ def resolveAliasesMaintainingSchema(ex: NamedExpression,
+ aliasMap: AttributeMap[Expression]):
NamedExpression = {
+ val ret = replaceAlias(ex, aliasMap)
+ ret match {
+ case namedEx: NamedExpression =>
+ if (namedEx.name != ex.name) {
+ Alias(ret, ex.name)(ex.exprId, ex.qualifier, Some(ex.metadata))
+ } else {
+ namedEx
+ }
+ case _ => Alias(ret, ex.name)(ex.exprId, ex.qualifier, Some(ex.metadata))
+ }
+ }
+
+ def collapseIntoOneAggregate(aggExprs: Seq[NamedExpression],
+ groupExprs: Seq[Expression],
+ childAgg: Aggregate): Aggregate = {
+
+ val aliasMap = AttributeMap(childAgg.aggregateExpressions.collect {
+ case a: Alias => (a.toAttribute, a.child)
+ })
+ val aliasResolvedAggExprs =
aggExprs.map(resolveAliasesMaintainingSchema(_, aliasMap))
+ val aliasResolvedGroupExprs = groupExprs.map(ex => replaceAlias(ex,
aliasMap))
+ Aggregate(aliasResolvedGroupExprs, aliasResolvedAggExprs, childAgg.child)
+ }
+
+ def apply(plan: LogicalPlan): LogicalPlan = {
+ plan transform {
+ // The query execution/optimization does not guarantee the expressions
are evaluated in order.
+ // We only can combine them if and only if both are deterministic.
+ case agg@Aggregate(groupExprs: Seq[Expression],
Review comment:
I think you can only collapse outer and inner Aggregate if the outer has
grouping expression among inner's grouping expressions?
For example:
```
SELECT sum(agg), min(a), b from (
SELECT sum(1) as agg, max(c) as maxC, a, b FROM testData2 GROUP BY a, b
) GROUP BY maxC
```
->
```
SELECT sum(1) as `sum(agg)`, max(c) as maxC, min(a) as `min(a)`, b from
testData2 GROUP BY maxC
```
?
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