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
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 41-2. pg_aggregate Columns
|aggfnoid||regproc||pg_proc.oid||pg_proc OID of the aggregate 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 value is null, the transition state value starts out null.|
New aggregate functions are registered with the CREATE AGGREGATE command. See Section 31.10 for more information about writing aggregate functions and the meaning of the transition functions, etc.
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