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
|aggfnoid||regproc||pg_proc.oid||pg_proc OID of the aggregate 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.