| PostgreSQL 9.1.14 Documentation | ||||
|---|---|---|---|---|
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   The catalog 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 data types, and
   other information that is similar to ordinary functions.
  
Table 45-2. 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) | 
| aggsortop | oid | pg_operator.oid | Associated sort operator (zero if none) | 
| aggtranstype | oid | pg_type.oid | Data 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 this field is null, the transition state value starts out null. | 
New aggregate functions are registered with the CREATE AGGREGATE command. See Section 35.10 for more information about writing aggregate functions and the meaning of the transition functions, etc.