52.2. pg_aggregate
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## 52.2. 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 52.2. pg_aggregate Columns

Column Type Description
`aggfnoid` regproc (references pg_proc.`oid`) pg_proc OID of the aggregate function
`aggkind` char Aggregate kind: `n` for “normal” aggregates, `o` for “ordered-set” aggregates, or `h` for “hypothetical-set” aggregates
`aggnumdirectargs` int2 Number of direct (non-aggregated) arguments of an ordered-set or hypothetical-set aggregate, counting a variadic array as one argument. If equal to `pronargs`, the aggregate must be variadic and the variadic array describes the aggregated arguments as well as the final direct arguments. Always zero for normal aggregates.
`aggtransfn` regproc (references pg_proc.`oid`) Transition function
`aggfinalfn` regproc (references pg_proc.`oid`) Final function (zero if none)
`aggcombinefn` regproc (references pg_proc.`oid`) Combine function (zero if none)
`aggserialfn` regproc (references pg_proc.`oid`) Serialization function (zero if none)
`aggdeserialfn` regproc (references pg_proc.`oid`) Deserialization function (zero if none)
`aggmtransfn` regproc (references pg_proc.`oid`) Forward transition function for moving-aggregate mode (zero if none)
`aggminvtransfn` regproc (references pg_proc.`oid`) Inverse transition function for moving-aggregate mode (zero if none)
`aggmfinalfn` regproc (references pg_proc.`oid`) Final function for moving-aggregate mode (zero if none)
`aggfinalextra` bool True to pass extra dummy arguments to `aggfinalfn`
`aggmfinalextra` bool True to pass extra dummy arguments to `aggmfinalfn`
`aggfinalmodify` char Whether `aggfinalfn` modifies the transition state value: `r` if it is read-only, `s` if the `aggtransfn` cannot be applied after the `aggfinalfn`, or `w` if it writes on the value
`aggmfinalmodify` char Like `aggfinalmodify`, but for the `aggmfinalfn`
`aggsortop` oid (references pg_operator.`oid`) Associated sort operator (zero if none)
`aggtranstype` oid (references pg_type.`oid`) Data type of the aggregate function's internal transition (state) data
`aggtransspace` int4 Approximate average size (in bytes) of the transition state data, or zero to use a default estimate
`aggmtranstype` oid (references pg_type.`oid`) Data type of the aggregate function's internal transition (state) data for moving-aggregate mode (zero if none)
`aggmtransspace` int4 Approximate average size (in bytes) of the transition state data for moving-aggregate mode, or zero to use a default estimate
`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.
`aggminitval` text The initial value of the transition state for moving-aggregate mode. 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 36.12 for more information about writing aggregate functions and the meaning of the transition functions, etc.

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