PostgreSQL 8.4.12 Documentation
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# 44.2. `pg_aggregate`

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 44-2. `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 34.10 for more information about writing aggregate functions and the meaning of the transition functions, etc.

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