Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/8973#discussion_r41348104
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/interfaces.scala
---
@@ -95,98 +92,192 @@ private[sql] case class AggregateExpression2(
override def toString: String =
s"(${aggregateFunction},mode=$mode,isDistinct=$isDistinct)"
}
-abstract class AggregateFunction2
- extends Expression with ImplicitCastInputTypes {
+/**
+ * AggregateFunction2 is the superclass of two aggregation function
interfaces:
+ *
+ * - [[ImperativeAggregate]] is for aggregation functions that are
specified in terms of
+ * initialize(), update(), and merge() functions that operate on
Row-based aggregation buffers.
+ * - [[ExpressionAggregate]] is for aggregation functions that are
specified using
+ * Catalyst expressions.
+ *
+ * In both interfaces, aggregates must define the schema
([[aggBufferSchema]]) and attributes
+ * ([[aggBufferAttributes]]) of an aggregation buffer which is used to
hold partial aggregate
+ * results. At runtime, multiple aggregate functions are evaluated by the
same operator using a
+ * combined aggregation buffer which concatenates the aggregation buffers
of the individual
+ * aggregate functions.
+ *
+ * Code which accepts [[AggregateFunction2]] instances should be prepared
to handle both types of
+ * aggregate functions.
+ */
+sealed abstract class AggregateFunction2 extends Expression with
ImplicitCastInputTypes {
/** An aggregate function is not foldable. */
final override def foldable: Boolean = false
+ /** The schema of the aggregation buffer. */
+ def aggBufferSchema: StructType
+
+ /** Attributes of fields in aggBufferSchema. */
+ def aggBufferAttributes: Seq[AttributeReference]
+
/**
- * The offset of this function's start buffer value in the
- * underlying shared mutable aggregation buffer.
- * For example, we have two aggregate functions `avg(x)` and `avg(y)`,
which share
- * the same aggregation buffer. In this shared buffer, the position of
the first
- * buffer value of `avg(x)` will be 0 and the position of the first
buffer value of `avg(y)`
- * will be 2.
+ * Attributes of fields in input aggregation buffers (immutable
aggregation buffers that are
+ * merged with mutable aggregation buffers in the merge() function or
merge expressions).
+ * These attributes are created automatically by cloning the
[[aggBufferAttributes]].
*/
- protected var mutableBufferOffset: Int = 0
-
- def withNewMutableBufferOffset(newMutableBufferOffset: Int): Unit = {
- mutableBufferOffset = newMutableBufferOffset
- }
+ def inputAggBufferAttributes: Seq[AttributeReference]
--- End diff --
@yhuai, why do we need to use different attributes to refer to the
aggregation buffer columns when they are embedded inside of a `[grouping
columns][partial agg columns]` result? Why can't you just re-use the same
attributes? It seems like this should significantly improve the code, but I
might be overlooking a reason why we need to do this.
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