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Make LATERAL implicit for functions in FROM.

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The SQL standard does not have general functions-in-FROM, but it does
allow UNNEST() there (see the <collection derived table> production),
and the semantics of that are defined to include lateral references.
So spec compliance requires allowing lateral references within UNNEST()
even without an explicit LATERAL keyword.  Rather than making UNNEST()
a special case, it seems best to extend this flexibility to any
function-in-FROM.  We'll still allow LATERAL to be written explicitly
for clarity's sake, but it's now a noise word in this context.

In theory this change could result in a change in behavior of existing
queries, by allowing what had been an outer reference in a function-in-FROM
to be captured by an earlier FROM-item at the same level.  However, all
pre-9.3 PG releases have a bug that causes them to match variable
references to earlier FROM-items in preference to outer references (and
then throw an error).  So no previously-working query could contain the
type of ambiguity that would risk a change of behavior.

Per a suggestion from Andrew Gierth, though I didn't use his patch.
This commit is contained in:
Tom Lane
2013-01-26 16:18:42 -05:00
parent 8865fe0ad3
commit 2378d79ab2
7 changed files with 101 additions and 46 deletions
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20
doc/src/sgml/queries.sgml
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@@ -717,14 +717,24 @@ SELECT *
</indexterm>
<para>
Subqueries and table functions appearing in <literal>FROM</> can be
Subqueries appearing in <literal>FROM</> can be
preceded by the key word <literal>LATERAL</>. This allows them to
reference columns provided by preceding <literal>FROM</> items.
(Without <literal>LATERAL</literal>, each <literal>FROM</> item is
(Without <literal>LATERAL</literal>, each subquery is
evaluated independently and so cannot cross-reference any other
<literal>FROM</> item.)
</para>
<para>
Table functions appearing in <literal>FROM</> can also be
preceded by the key word <literal>LATERAL</>, but for functions the
key word is optional; the function's arguments can contain references
to columns provided by preceding <literal>FROM</> items in any case.
</para>
<para>
A <literal>LATERAL</literal> item can appear at top level in the
<literal>FROM</> list, or within a <literal>JOIN</> tree; in the latter
<literal>FROM</> list, or within a <literal>JOIN</> tree. In the latter
case it can also refer to any items that are on the left-hand side of a
<literal>JOIN</> that it is on the right-hand side of.
</para>
@@ -770,7 +780,9 @@ FROM polygons p1 CROSS JOIN LATERAL vertices(p1.poly) v1,
polygons p2 CROSS JOIN LATERAL vertices(p2.poly) v2
WHERE (v1 &lt;-&gt; v2) &lt; 10 AND p1.id != p2.id;
</programlisting>
or in several other equivalent formulations.
or in several other equivalent formulations. (As already mentioned,
the <literal>LATERAL</> key word is unnecessary in this example, but
we use it for clarity.)
</para>
<para>