## 52.2. `pg_aggregate`
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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`](catalog-pg-proc.html). 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`](catalog-pg-proc.html).`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`](catalog-pg-proc.html).`oid`)
Transition function |
| `aggfinalfn` `regproc` (references [`pg_proc`](catalog-pg-proc.html).`oid`)
Final function (zero if none) |
| `aggcombinefn` `regproc` (references [`pg_proc`](catalog-pg-proc.html).`oid`)
Combine function (zero if none) |
| `aggserialfn` `regproc` (references [`pg_proc`](catalog-pg-proc.html).`oid`)
Serialization function (zero if none) |
| `aggdeserialfn` `regproc` (references [`pg_proc`](catalog-pg-proc.html).`oid`)
Deserialization function (zero if none) |
| `aggmtransfn` `regproc` (references [`pg_proc`](catalog-pg-proc.html).`oid`)
Forward transition function for moving-aggregate mode (zero if none) |
| `aggminvtransfn` `regproc` (references [`pg_proc`](catalog-pg-proc.html).`oid`)
Inverse transition function for moving-aggregate mode (zero if none) |
| `aggmfinalfn` `regproc` (references [`pg_proc`](catalog-pg-proc.html).`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`](catalog-pg-operator.html).`oid`)
Associated sort operator (zero if none) |
| `aggtranstype` `oid` (references [`pg_type`](catalog-pg-type.html).`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`](catalog-pg-type.html).`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`](sql-createaggregate.html) command. See [Section 38.12](xaggr.html) for more information about writing aggregate functions and the meaning of the transition functions, etc.