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.
Each entry in pg_aggregate is an
extension of an entry in pg_proc. The
pg_proc entry carries the aggregate's
name, input and output datatypes, and other information that is
similar to ordinary functions.
Table 3-2. pg_aggregate Columns
| Name | Type | References | Description |
|---|---|---|---|
| aggfnoid | regproc | pg_proc.oid | pg_proc OID of the aggregate function |
| aggtransfn | regproc | pg_proc.oid | Transition function |
| aggfinalfn | regproc | pg_proc.oid | Final function (zero if none) |
| aggtranstype | oid | pg_type.oid | The type of the aggregate function's internal transition (state) data |
| 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.