pg_aggregate

PostgreSQL 7.3.4 Documentation
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# 3.2. pg_aggregate

`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.

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