clauses(s) considered after
    that.  In all other cases, <quote>left to right</quote> means the order
    in which the expressions appear in the query text.
   </para>
  </footnote>
If the candidate type can be implicitly converted to the other type,
but not vice-versa, select the other type as the new candidate type.
Then continue considering the remaining inputs.  If, at any stage of this
process, a preferred type is selected, stop considering additional
inputs.
</para>
</step>

<step performance="required">
<para>
Convert all inputs to the final candidate type.  Fail if there is not an
implicit conversion from a given input type to the candidate type.
</para>
</step>
</procedure>

<para>
Some examples follow.
</para>

<example>
<title>Type Resolution with Underspecified Types in a Union</title>

<para>
<screen>
SELECT text 'a' AS "text" UNION SELECT 'b';

 text
------
 a
 b
(2 rows)
</screen>
Here, the unknown-type literal <literal>'b'</literal> will be resolved to type <type>text</type>.
</para>
</example>

<example>
<title>Type Resolution in a Simple Union</title>

<para>
<screen>
SELECT 1.2 AS "numeric" UNION SELECT 1;

 numeric
---------
       1
     1.2
(2 rows)
</screen>
The literal <literal>1.2</literal> is of type <type>numeric</type>,
and the <type>integer</type> value <literal>1</literal> can be cast implicitly to
<type>numeric</type>, so that type is used.
</para>
</example>

<example>
<title>Type Resolution in a Transposed Union</title>

<para>
<screen>
SELECT 1 AS "real" UNION SELECT CAST('2.2' AS REAL);

 real
------
    1
  2.2
(2 rows)
</screen>
Here, since type <type>real</type> cannot be implicitly cast to <type>integer</type>,
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>