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><A
NAME="CATALOG-PG-AGGREGATE"
>44.2. <TT
CLASS="STRUCTNAME"
>pg_aggregate</TT
></A
></H1
><A
NAME="AEN67971"
></A
><P
> The catalog <TT
CLASS="STRUCTNAME"
>pg_aggregate</TT
> 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
<CODE
CLASS="FUNCTION"
>sum</CODE
>, <CODE
CLASS="FUNCTION"
>count</CODE
>, and
<CODE
CLASS="FUNCTION"
>max</CODE
>. Each entry in
<TT
CLASS="STRUCTNAME"
>pg_aggregate</TT
> is an extension of an entry
in <TT
CLASS="STRUCTNAME"
>pg_proc</TT
>. The <TT
CLASS="STRUCTNAME"
>pg_proc</TT
>
entry carries the aggregate's name, input and output data types, and
other information that is similar to ordinary functions.
</P
><DIV
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><A
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></A
><P
><B
>Table 44-2. <TT
CLASS="STRUCTNAME"
>pg_aggregate</TT
> Columns</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Type</TH
><TH
>References</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>aggfnoid</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
> OID of the aggregate function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>aggtransfn</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
>Transition function</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>aggfinalfn</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></TD
><TD
>Final function (zero if none)</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>aggsortop</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-operator.html"
><TT
CLASS="STRUCTNAME"
>pg_operator</TT
></A
>.oid</TT
></TD
><TD
>Associated sort operator (zero if none)</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>aggtranstype</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-type.html"
><TT
CLASS="STRUCTNAME"
>pg_type</TT
></A
>.oid</TT
></TD
><TD
>Data type of the aggregate function's internal transition (state) data</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>agginitval</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
> </TD
><TD
> 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
</TD
></TR
></TBODY
></TABLE
></DIV
><P
> New aggregate functions are registered with the <A
HREF="sql-createaggregate.html"
><I
>CREATE AGGREGATE</I
></A
>
command. See <A
HREF="xaggr.html"
>Section 34.10</A
> for more information about
writing aggregate functions and the meaning of the transition
functions, etc.
</P
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