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 the 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
|aggname||name||Name of the aggregate function|
|aggowner||int4||pg_shadow.usesysid||Owner (creator) of the aggregate function|
|aggtransfn||regproc (function)||Transition function|
|aggfinalfn||regproc (function)||Final function|
|aggbasetype||oid||pg_type.oid||The type on which this function operates when invoked from SQL|
|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 which will be cast to the type of aggtranstype.|
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 aggname are the composite primary key.