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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FUNCTIONS-AGGREGATE"
>9.15. Aggregate Functions</A
></H1
><A
NAME="AEN12307"
></A
><P
> <I
CLASS="FIRSTTERM"
>Aggregate functions</I
> compute a single result
value from a set of input values. <A
HREF="functions-aggregate.html#FUNCTIONS-AGGREGATE-TABLE"
>Table 9-37</A
> shows the built-in aggregate
functions. The special syntax considerations for aggregate
functions are explained in <A
HREF="sql-expressions.html#SYNTAX-AGGREGATES"
>Section 4.2.7</A
>.
Consult <A
HREF="tutorial-agg.html"
>Section 2.7</A
> for additional introductory
information.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-AGGREGATE-TABLE"
></A
><P
><B
>Table 9-37. Aggregate Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><COL><THEAD
><TR
><TH
>Function</TH
><TH
>Argument Type</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <A
NAME="AEN12327"
></A
>
<CODE
CLASS="FUNCTION"
>avg(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
>
</TD
><TD
> <TT
CLASS="TYPE"
>smallint</TT
>, <TT
CLASS="TYPE"
>integer</TT
>,
<TT
CLASS="TYPE"
>bigint</TT
>, <TT
CLASS="TYPE"
>real</TT
>, <TT
CLASS="TYPE"
>double
precision</TT
>, <TT
CLASS="TYPE"
>numeric</TT
>, or <TT
CLASS="TYPE"
>interval</TT
>
</TD
><TD
> <TT
CLASS="TYPE"
>numeric</TT
> for any integer type argument,
<TT
CLASS="TYPE"
>double precision</TT
> for a floating-point argument,
otherwise the same as the argument data type
</TD
><TD
>the average (arithmetic mean) of all input values</TD
></TR
><TR
><TD
> <A
NAME="AEN12345"
></A
>
<CODE
CLASS="FUNCTION"
>bit_and(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
>
</TD
><TD
> <TT
CLASS="TYPE"
>smallint</TT
>, <TT
CLASS="TYPE"
>integer</TT
>, <TT
CLASS="TYPE"
>bigint</TT
>, or
<TT
CLASS="TYPE"
>bit</TT
>
</TD
><TD
> same as argument data type
</TD
><TD
>the bitwise AND of all non-null input values, or null if none</TD
></TR
><TR
><TD
> <A
NAME="AEN12358"
></A
>
<CODE
CLASS="FUNCTION"
>bit_or(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
>
</TD
><TD
> <TT
CLASS="TYPE"
>smallint</TT
>, <TT
CLASS="TYPE"
>integer</TT
>, <TT
CLASS="TYPE"
>bigint</TT
>, or
<TT
CLASS="TYPE"
>bit</TT
>
</TD
><TD
> same as argument data type
</TD
><TD
>the bitwise OR of all non-null input values, or null if none</TD
></TR
><TR
><TD
> <A
NAME="AEN12371"
></A
>
<CODE
CLASS="FUNCTION"
>bool_and(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
>
</TD
><TD
> <TT
CLASS="TYPE"
>bool</TT
>
</TD
><TD
> <TT
CLASS="TYPE"
>bool</TT
>
</TD
><TD
>true if all input values are true, otherwise false</TD
></TR
><TR
><TD
> <A
NAME="AEN12382"
></A
>
<CODE
CLASS="FUNCTION"
>bool_or(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
>
</TD
><TD
> <TT
CLASS="TYPE"
>bool</TT
>
</TD
><TD
> <TT
CLASS="TYPE"
>bool</TT
>
</TD
><TD
>true if at least one input value is true, otherwise false</TD
></TR
><TR
><TD
><CODE
CLASS="FUNCTION"
>count(*)</CODE
></TD
><TD
> </TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
>number of input values</TD
></TR
><TR
><TD
><CODE
CLASS="FUNCTION"
>count(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
></TD
><TD
>any</TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
> number of input values for which the value of <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> is not null
</TD
></TR
><TR
><TD
> <A
NAME="AEN12409"
></A
>
<CODE
CLASS="FUNCTION"
>every(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
>
</TD
><TD
> <TT
CLASS="TYPE"
>bool</TT
>
</TD
><TD
> <TT
CLASS="TYPE"
>bool</TT
>
</TD
><TD
>equivalent to <CODE
CLASS="FUNCTION"
>bool_and</CODE
></TD
></TR
><TR
><TD
><CODE
CLASS="FUNCTION"
>max(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
></TD
><TD
>any numeric, string, or date/time type</TD
><TD
>same as argument type</TD
><TD
> maximum value of <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> across all input
values
</TD
></TR
><TR
><TD
><CODE
CLASS="FUNCTION"
>min(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
></TD
><TD
>any numeric, string, or date/time type</TD
><TD
>same as argument type</TD
><TD
> minimum value of <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> across all input
values
</TD
></TR
><TR
><TD
> <A
NAME="AEN12437"
></A
>
<CODE
CLASS="FUNCTION"
>stddev(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
>
</TD
><TD
> <TT
CLASS="TYPE"
>smallint</TT
>, <TT
CLASS="TYPE"
>integer</TT
>,
<TT
CLASS="TYPE"
>bigint</TT
>, <TT
CLASS="TYPE"
>real</TT
>, <TT
CLASS="TYPE"
>double
precision</TT
>, or <TT
CLASS="TYPE"
>numeric</TT
>
</TD
><TD
> <TT
CLASS="TYPE"
>double precision</TT
> for floating-point arguments,
otherwise <TT
CLASS="TYPE"
>numeric</TT
>
</TD
><TD
>sample standard deviation of the input values</TD
></TR
><TR
><TD
><CODE
CLASS="FUNCTION"
>sum(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)</CODE
></TD
><TD
> <TT
CLASS="TYPE"
>smallint</TT
>, <TT
CLASS="TYPE"
>integer</TT
>,
<TT
CLASS="TYPE"
>bigint</TT
>, <TT
CLASS="TYPE"
>real</TT
>, <TT
CLASS="TYPE"
>double
precision</TT
>, <TT
CLASS="TYPE"
>numeric</TT
>, or
<TT
CLASS="TYPE"
>interval</TT
>
</TD
><TD
> <TT
CLASS="TYPE"
>bigint</TT
> for <TT
CLASS="TYPE"
>smallint</TT
> or
<TT
CLASS="TYPE"
>integer</TT
> arguments, <TT
CLASS="TYPE"
>numeric</TT
> for
<TT
CLASS="TYPE"
>bigint</TT
> arguments, <TT
CLASS="TYPE"
>double precision</TT
>
for floating-point arguments, otherwise the same as the
argument data type
</TD
><TD
>sum of <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> across all input values</TD
></TR
><TR
><TD
> <A
NAME="AEN12475"
></A
>
<CODE
CLASS="FUNCTION"
>variance</CODE
>(<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>)
</TD
><TD
> <TT
CLASS="TYPE"
>smallint</TT
>, <TT
CLASS="TYPE"
>integer</TT
>,
<TT
CLASS="TYPE"
>bigint</TT
>, <TT
CLASS="TYPE"
>real</TT
>, <TT
CLASS="TYPE"
>double
precision</TT
>, or <TT
CLASS="TYPE"
>numeric</TT
>
</TD
><TD
> <TT
CLASS="TYPE"
>double precision</TT
> for floating-point arguments,
otherwise <TT
CLASS="TYPE"
>numeric</TT
>
</TD
><TD
>sample variance of the input values (square of the sample standard deviation)</TD
></TR
></TBODY
></TABLE
></DIV
><P
> It should be noted that except for <CODE
CLASS="FUNCTION"
>count</CODE
>,
these functions return a null value when no rows are selected. In
particular, <CODE
CLASS="FUNCTION"
>sum</CODE
> of no rows returns null, not
zero as one might expect. The <CODE
CLASS="FUNCTION"
>coalesce</CODE
> function may be
used to substitute zero for null when necessary.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><A
NAME="AEN12495"
></A
><A
NAME="AEN12497"
></A
><P
><B
>Note: </B
> Boolean aggregates <CODE
CLASS="FUNCTION"
>bool_and</CODE
> and
<CODE
CLASS="FUNCTION"
>bool_or</CODE
> correspond to standard SQL aggregates
<CODE
CLASS="FUNCTION"
>every</CODE
> and <CODE
CLASS="FUNCTION"
>any</CODE
> or
<CODE
CLASS="FUNCTION"
>some</CODE
>.
As for <CODE
CLASS="FUNCTION"
>any</CODE
> and <CODE
CLASS="FUNCTION"
>some</CODE
>,
it seems that there is an ambiguity built into the standard syntax:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT b1 = ANY((SELECT b2 FROM t2 ...)) FROM t1 ...;</PRE
><P>
Here <CODE
CLASS="FUNCTION"
>ANY</CODE
> can be considered both as leading
to a subquery or as an aggregate if the select expression returns 1 row.
Thus the standard name cannot be given to these aggregates.
</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Users accustomed to working with other SQL database management
systems may be surprised by the performance characteristics of
certain aggregate functions in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> when the aggregate is
applied to the entire table (in other words, no
<TT
CLASS="LITERAL"
>WHERE</TT
> clause is specified). In particular, a
query like
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT min(col) FROM sometable;</PRE
><P>
will be executed by <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> using a
sequential scan of the entire table. Other database systems may
optimize queries of this form to use an index on the column, if
one is available. Similarly, the aggregate functions
<CODE
CLASS="FUNCTION"
>max()</CODE
> and <CODE
CLASS="FUNCTION"
>count()</CODE
> always
require a sequential scan if applied to the entire table in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> cannot easily implement this
optimization because it also allows for user-defined aggregate
queries. Since <CODE
CLASS="FUNCTION"
>min()</CODE
>,
<CODE
CLASS="FUNCTION"
>max()</CODE
>, and <CODE
CLASS="FUNCTION"
>count()</CODE
> are
defined using a generic API for aggregate functions, there is no
provision for special-casing the execution of these functions
under certain circumstances.
</P
><P
> Fortunately, there is a simple workaround for
<CODE
CLASS="FUNCTION"
>min()</CODE
> and <CODE
CLASS="FUNCTION"
>max()</CODE
>. The
query shown below is equivalent to the query above, except that it
can take advantage of a B-tree index if there is one present on
the column in question.
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT col FROM sometable ORDER BY col ASC LIMIT 1;</PRE
><P>
A similar query (obtained by substituting <TT
CLASS="LITERAL"
>DESC</TT
>
for <TT
CLASS="LITERAL"
>ASC</TT
> in the query above) can be used in the
place of <CODE
CLASS="FUNCTION"
>max()</CODE
>.
</P
><P
> Unfortunately, there is no similarly trivial query that can be
used to improve the performance of <CODE
CLASS="FUNCTION"
>count()</CODE
>
when applied to the entire table.
</P
></BLOCKQUOTE
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