| Документация по PostgreSQL 9.1.1 | ||||
|---|---|---|---|---|
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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.