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| -rw-r--r-- | doc/src/sgml/typeconv.sgml | 30 |
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diff --git a/doc/src/sgml/typeconv.sgml b/doc/src/sgml/typeconv.sgml index cd7de8fe3fa..ed4be4fcfcc 100644 --- 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"> |