51.2. 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 51.2.
pg_aggregate
Columns
Column Type Description 


Aggregate kind:

Number of direct (nonaggregated) arguments of an orderedset or
hypotheticalset aggregate, counting a variadic array as one argument.
If equal to

Transition function 
Final function (zero if none) 
Combine function (zero if none) 
Serialization function (zero if none) 
Deserialization function (zero if none) 
Forward transition function for movingaggregate mode (zero if none) 
Inverse transition function for movingaggregate mode (zero if none) 
Final function for movingaggregate mode (zero if none) 
True to pass extra dummy arguments to

True to pass extra dummy arguments to

Whether

Like

Associated sort operator (zero if none) 
Data type of the aggregate function's internal transition (state) data 
Approximate average size (in bytes) of the transition state data, or zero to use a default estimate 
Data type of the aggregate function's internal transition (state) data for movingaggregate mode (zero if none) 
Approximate average size (in bytes) of the transition state data for movingaggregate mode, or zero to use a default estimate 
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. 
The initial value of the transition state for movingaggregate 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 37.12 for more information about writing aggregate functions and the meaning of the transition functions, etc.