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Doc: improve documentation about composite-value usage.

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Create a section specifically for the syntactic rules around whole-row
variable usage, such as expansion of "foo.*".  This was previously
documented only haphazardly, with some critical info buried in
unexpected places like xfunc-sql-composite-functions.  Per repeated
questions in different mailing lists.

Discussion: <16288.1479610770@sss.pgh.pa.us>
This commit is contained in:
Tom Lane
2016-11-22 17:56:16 -05:00
parent 9a1d0af4ad
commit e1320266ed
4 changed files with 231 additions and 72 deletions
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16
doc/src/sgml/syntax.sgml
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@ -1449,12 +1449,13 @@ $1.somecolumn
</para>
<para>
In a select list (see <xref linkend="queries-select-lists">), you
can ask for all fields of a composite value by
You can ask for all fields of a composite value by
writing <literal>.*</literal>:
<programlisting>
(compositecol).*
</programlisting>
This notation behaves differently depending on context;
see <xref linkend="rowtypes-usage"> for details.
</para>
</sect2>
@ -1531,7 +1532,7 @@ sqrt(2)
interchangeable. This behavior is not SQL-standard but is provided
in <productname>PostgreSQL</> because it allows use of functions to
emulate <quote>computed fields</>. For more information see
<xref linkend="xfunc-sql-composite-functions">.
<xref linkend="rowtypes-usage">.
</para>
</note>
</sect2>
@ -2291,7 +2292,8 @@ SELECT ROW(1,2.5,'this is a test');
<replaceable>rowvalue</replaceable><literal>.*</literal>,
which will be expanded to a list of the elements of the row value,
just as occurs when the <literal>.*</> syntax is used at the top level
of a <command>SELECT</> list. For example, if table <literal>t</> has
of a <command>SELECT</> list (see <xref linkend="rowtypes-usage">).
For example, if table <literal>t</> has
columns <literal>f1</> and <literal>f2</>, these are the same:
<programlisting>
SELECT ROW(t.*, 42) FROM t;
@ -2302,9 +2304,9 @@ SELECT ROW(t.f1, t.f2, 42) FROM t;
<note>
<para>
Before <productname>PostgreSQL</productname> 8.2, the
<literal>.*</literal> syntax was not expanded, so that writing
<literal>ROW(t.*, 42)</> created a two-field row whose first field
was another row value. The new behavior is usually more useful.
<literal>.*</literal> syntax was not expanded in row constructors, so
that writing <literal>ROW(t.*, 42)</> created a two-field row whose first
field was another row value. The new behavior is usually more useful.
If you need the old behavior of nested row values, write the inner
row value without <literal>.*</literal>, for instance
<literal>ROW(t, 42)</>.