AggregateExpression¶

AggregateExpression is an unevaluable expression that acts as a container (wrapper) for an AggregateFunction.

Creating Instance¶

AggregateExpression takes the following to be created:

• AggregateFunction
• AggregateMode
• isDistinct flag
• Aggregate Filter Expression
• Result ExprId

AggregateExpression is created using apply utility.

Aggregate Filter Expression¶

AggregateExpression can be given an Expression for an aggregate filter.

The filter is assumed undefined by default when AggregateExpression is created.

A filter is used in Partial and Complete modes only (cf. AggUtils).

AggregationIterator initializes predicates with AggregateExpressions with filters when requested to generateProcessRow.

AggregateMode¶

AggregateExpression is given an AggregateMode when created.

• For PartialMerge or Final modes, the input to the AggregateFunction is immutable input aggregation buffers, and the actual children of the AggregateFunction are not used

• AggregateExpressions of an AggregationIterator cannot have more than 2 distinct modes nor the modes be among Partial and PartialMerge or Final and Complete mode pairs

• Partial and Complete or PartialMerge and Final pairs are supported

Complete¶

No prefix (in toString)

Used when:

• ObjectAggregationIterator is requested for the mergeAggregationBuffers
• TungstenAggregationIterator is requested for the switchToSortBasedAggregation
• others

Final¶

No prefix (in toString)

Partial¶

• Partial aggregation
• partial_ prefix (in toString)

PartialMerge¶

• merge_ prefix (in toString)

Creating AggregateExpression¶

apply(
  aggregateFunction: AggregateFunction,
  mode: AggregateMode,
  isDistinct: Boolean,
  filter: Option[Expression] = None): AggregateExpression

apply creates an AggregateExpression with an auto-generated ExprId.

apply is used when:

• AggregateFunction is requested to toAggregateExpression
• AggUtils is requested to planAggregateWithOneDistinct

Human-Friendly Textual Representation¶

toString: String

toString returns the following text:

[prefix][name]([args]) FILTER (WHERE [predicate])

toString converts the mode to a prefix.

toString requests the AggregateFunction for the toAggString (with the isDistinct flag).

In the end, toString adds FILTER (WHERE [predicate]) based on the optional filter expression.