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
Name  Type  References  Description 

aggfnoid

regproc


pg_proc
OID of the aggregate function

aggkind

char

Aggregate kind:
n
for
"
normal
"
aggregates,
o
for
"
orderedset
"
aggregates, or
h
for
"
hypotheticalset
"
aggregates


aggnumdirectargs

int2

Number of direct (nonaggregated) arguments of an orderedset or
hypotheticalset 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


Transition function 
aggfinalfn

regproc


Final function (zero if none) 
aggcombinefn

regproc


Combine function (zero if none) 
aggserialfn

regproc


Serialization function (zero if none) 
aggdeserialfn

regproc


Deserialization function (zero if none) 
aggmtransfn

regproc


Forward transition function for movingaggregate mode (zero if none) 
aggminvtransfn

regproc


Inverse transition function for movingaggregate mode (zero if none) 
aggmfinalfn

regproc


Final function for movingaggregate 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 readonly,
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


Associated sort operator (zero if none) 
aggtranstype

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


Data type of the aggregate function's internal transition (state) data for movingaggregate mode (zero if none) 
aggmtransspace

int4

Approximate average size (in bytes) of the transition state data for movingaggregate 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 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 38.11 for more information about writing aggregate functions and the meaning of the transition functions, etc.