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Clarify documentation of handling of null arguments for aggregates.

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Per discussion.
This commit is contained in:
Tom Lane
2010-09-01 18:22:35 +00:00
parent 41c8287de0
commit 43e48de4a5
2 changed files with 18 additions and 12 deletions
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4
doc/src/sgml/func.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.521.2.4 2010/08/17 04:37:16 petere Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.521.2.5 2010/09/01 18:22:35 tgl Exp $ -->
<chapter id="functions">
<title>Functions and Operators</title>
@ -9760,7 +9760,7 @@ SELECT NULLIF(value, '(none)') ...
<entry>
array of the argument type
</entry>
<entry>input values concatenated into an array</entry>
<entry>input values, including nulls, concatenated into an array</entry>
</row>
<row>

26
doc/src/sgml/syntax.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.147.2.5 2010/08/13 01:12:51 rhaas Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.147.2.6 2010/09/01 18:22:35 tgl Exp $ -->
<chapter id="sql-syntax">
<title>SQL Syntax</title>
@ -1542,23 +1542,29 @@ sqrt(2)
<para>
The first form of aggregate expression invokes the aggregate
across all input rows for which the given expression(s) yield
non-null values. (Actually, it is up to the aggregate function
whether to ignore null values or not &mdash; but all the standard ones do.)
once for each input row.
The second form is the same as the first, since
<literal>ALL</literal> is the default. The third form invokes the
aggregate for all distinct values of the expressions found
in the input rows (ignoring nulls if the function chooses to do so).
The last form invokes the aggregate once for
each input row regardless of null or non-null values; since no
<literal>ALL</literal> is the default.
The third form invokes the aggregate once for each distinct value
of the expression (or distinct set of values, for multiple expressions)
found in the input rows.
The last form invokes the aggregate once for each input row; since no
particular input value is specified, it is generally only useful
for the <function>count(*)</function> aggregate function.
</para>
<para>
Most aggregate functions ignore null inputs, so that rows in which
one or more of the expression(s) yield null are discarded. This
can be assumed to be true, unless otherwise specified, for all
built-in aggregates.
</para>
<para>
For example, <literal>count(*)</literal> yields the total number
of input rows; <literal>count(f1)</literal> yields the number of
input rows in which <literal>f1</literal> is non-null;
input rows in which <literal>f1</literal> is non-null, since
<function>count</> ignores nulls; and
<literal>count(distinct f1)</literal> yields the number of
distinct non-null values of <literal>f1</literal>.
</para>