Aggregate Functions¶
Aggregate functions are functions that define an operation which consumes values from multiple records to a produce a single output. Aggregate functions in SQL are typically used in GROUP BY functions. Aggregate functions are similar to scalar functions and function signatures with a small set of different properties.
Aggregate function signatures contain all the properties defined for scalar functions. Additionally, they contain the properties below:
| Property | Description | Required |
|---|---|---|
| Inherits | All properties defined for scalar function. | N/A |
| Ordered | Whether this aggregation function should allow user ordering. | Optional, defaults to false |
| Maximum set size | Maximum allowed set size as an unsigned integer. | Optional, defaults to unlimited |
| Decomposable | Whether the function can be executed in one or more intermediate steps. Valid options are: NONE, ONE, MANY, describing how intermediate steps can be taken. | Optional, defaults to NONE |
| Intermediate Output Type | If the function is decomposable, represents the intermediate output type that is used, if the function is defined as either ONE or MANY decomposable. Will be a struct in many cases. | Required for ONE and MANY. |
| Invocation | Whether the function uses all or only distinct values in the aggregation calculation. Valid options are: ALL, DISTINCT. | Optional, defaults to ALL |
Aggregate Binding¶
When binding an aggregate function, the binding must include the following additional properties beyond the standard scalar binding properties:
| Property | Description |
|---|---|
| Phase | Describes the input type of the data: [INITIAL_TO_INTERMEDIATE, INTERMEDIATE_TO_INTERMEDIATE, INITIAL_TO_RESULT, INTERMEDIATE_TO_RESULT] describing what portion of the operation is required. For functions that are NOT decomposable, the only valid option will be INITIAL_TO_RESULT. |
| Ordering | One or more ordering keys along with key order (ASC|DESC|NULL FIRST, etc.), declared similar to the sort keys in an ORDER BY relational operation. Only allowed in cases where the function signature supports ordering. |