pg_aggregate stores information
about aggregate functions. An aggregate function is a function
that operates on a set of values (typically one column from each
row that matches a query condition) and returns a single value
computed from all these values. Typical aggregate functions are
sum, count, and max.
Table 3-2. pg_aggregate Columns
| Name | Type | References | Description |
|---|---|---|---|
| aggname | name | Name of the aggregate function | |
| aggowner | int4 | pg_shadow.usesysid | Owner (creator) of the aggregate function |
| aggtransfn | regproc (function) | pg_proc.oid | Transition function |
| aggfinalfn | regproc (function) | pg_proc.oid | Final function |
| aggbasetype | oid | pg_type.oid | The input datatype for this aggregate function |
| aggtranstype | oid | pg_type.oid | The type of the aggregate function's internal transition (state) data |
| aggfinaltype | oid | pg_type.oid | The type of the result |
| agginitval | text | The initial value of the transition state. This is a text field containing the initial value in its external string representation. If the field is NULL, the transition state value starts out NULL. |
New aggregate functions are registered with the CREATE AGGREGATE command. See the Programmer's Guide for more information about writing aggregate functions and the meaning of the transition functions, etc.
An aggregate function is identified through name and argument type. Hence aggname and aggbasetype are the composite primary key.