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Allow empty target list in SELECT.

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This fixes a problem noted as a followup to bug #8648: if a query has a
semantically-empty target list, e.g. SELECT * FROM zero_column_table,
ruleutils.c will dump it as a syntactically-empty target list, which was
not allowed.  There doesn't seem to be any reliable way to fix this by
hacking ruleutils (note in particular that the originally zero-column table
might since have had columns added to it); and even if we had such a fix,
it would do nothing for existing dump files that might contain bad syntax.
The best bet seems to be to relax the syntactic restriction.

Also, add parse-analysis errors for SELECT DISTINCT with no columns (after
*-expansion) and RETURNING with no columns.  These cases previously
produced unexpected behavior because the parsed Query looked like it had
no DISTINCT or RETURNING clause, respectively.  If anyone ever offers
a plausible use-case for this, we could work a bit harder on making the
situation distinguishable.

Arguably this is a bug fix that should be back-patched, but I'm worried
that there may be client apps or PLs that expect "SELECT ;" to throw a
syntax error.  The issue doesn't seem important enough to risk changing
behavior in minor releases.
This commit is contained in:
Tom Lane
2013-12-14 20:23:26 -05:00
parent c03ad5602f
commit 1b4f7f93b4
6 changed files with 73 additions and 33 deletions
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28
doc/src/sgml/ref/select.sgml
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@@ -34,7 +34,7 @@ PostgreSQL documentation
<synopsis>
[ WITH [ RECURSIVE ] <replaceable class="parameter">with_query</replaceable> [, ...] ]
SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replaceable> [, ...] ) ] ]
* | <replaceable class="parameter">expression</replaceable> [ [ AS ] <replaceable class="parameter">output_name</replaceable> ] [, ...]
[ * | <replaceable class="parameter">expression</replaceable> [ [ AS ] <replaceable class="parameter">output_name</replaceable> ] [, ...] ]
[ FROM <replaceable class="parameter">from_item</replaceable> [, ...] ]
[ WHERE <replaceable class="parameter">condition</replaceable> ]
[ GROUP BY <replaceable class="parameter">expression</replaceable> [, ...] ]
@@ -1740,13 +1740,27 @@ SELECT 2+2;
following query is invalid:
<programlisting>
SELECT distributors.* WHERE distributors.name = 'Westward';
</programlisting><productname>PostgreSQL</productname> releases prior to
</programlisting>
<productname>PostgreSQL</productname> releases prior to
8.1 would accept queries of this form, and add an implicit entry
to the query's <literal>FROM</literal> clause for each table
referenced by the query. This is no longer allowed.
</para>
</refsect2>
<refsect2>
<title>Empty <literal>SELECT</literal> Lists</title>
<para>
The list of output expressions after <literal>SELECT</literal> can be
empty, producing a zero-column result table.
This is not valid syntax according to the SQL standard.
<productname>PostgreSQL</productname> allows it to be consistent with
allowing zero-column tables.
However, an empty list is not allowed when <literal>DISTINCT</> is used.
</para>
</refsect2>
<refsect2>
<title>Omitting the <literal>AS</literal> Key Word</title>
@@ -1809,10 +1823,6 @@ SELECT distributors.* WHERE distributors.name = 'Westward';
<productname>PostgreSQL</productname> treats <literal>UNNEST()</> the
same as other set-returning functions.
</para>
<para>
<literal>ROWS FROM( ... )</> is an extension of the SQL standard.
</para>
</refsect2>
<refsect2>
@@ -1910,9 +1920,13 @@ SELECT distributors.* WHERE distributors.name = 'Westward';
<title>Nonstandard Clauses</title>
<para>
The clause <literal>DISTINCT ON</literal> is not defined in the
<literal>DISTINCT ON ( ... )</literal> is an extension of the
SQL standard.
</para>
<para>
<literal>ROWS FROM( ... )</> is an extension of the SQL standard.
</para>
</refsect2>
</refsect1>
</refentry>