Doc: add example of type resolution in nested UNIONs.

Section 10.5 didn't say explicitly that multiple UNIONs are resolved
pairwise.  Since the resolution algorithm is described as taking any
number of inputs, readers might well think that a query like
"select x union select y union select z" would be resolved by
considering x, y, and z in one resolution step.  But that's not what
happens (and I think that behavior is per SQL spec).  Add an example
clarifying this point.

Per bug #15129 from Philippe Beaudoin.

Discussion: https://postgr.es/m/152196085023.32649.9916472370480121694@wrigleys.postgresql.org

diff --git a/doc/src/sgml/typeconv.sgml b/doc/src/sgml/typeconv.sgml
index cd7de8fe3fa6d4d5ac4d9a0a8f97ddfb4288dc56..ed4be4fcfcc0204784ccef254f7f927c8c9d0e5c 100644 (file)
--- a/doc/src/sgml/typeconv.sgml
+++ b/doc/src/sgml/typeconv.sgml
@@ -1077,6 +1077,36 @@ but <type>integer</type> can be implicitly cast to <type>real</type>, the union
 result type is resolved as <type>real</type>.
 </para>
 </example>
+
+<example>
+<title>Type Resolution in a Nested Union</title>
+
+<para>
+<screen>
+SELECT NULL UNION SELECT NULL UNION SELECT 1;
+
+ERROR:  UNION types text and integer cannot be matched
+</screen>
+This failure occurs because <productname>PostgreSQL</productname> treats
+multiple <literal>UNION</literal>s as a nest of pairwise operations;
+that is, this input is the same as
+<screen>
+(SELECT NULL UNION SELECT NULL) UNION SELECT 1;
+</screen>
+The inner <literal>UNION</literal> is resolved as emitting
+type <type>text</type>, according to the rules given above.  Then the
+outer <literal>UNION</literal> has inputs of types <type>text</type>
+and <type>integer</type>, leading to the observed error.  The problem
+can be fixed by ensuring that the leftmost <literal>UNION</literal>
+has at least one input of the desired result type.
+</para>
+
+<para>
+<literal>INTERSECT</literal> and <literal>EXCEPT</literal> operations are
+likewise resolved pairwise.  However, the other constructs described in this
+section consider all of their inputs in one resolution step.
+</para>
+</example>
 </sect1>
 
 <sect1 id="typeconv-select">