linkend="sql-createcast"/>).
Alternatively, if the expression is an unknown-type literal, the contents of
the literal string will be fed to the input conversion routine for the target
type.
</para>
</step>

<step performance="required">
<para>
Check to see if there is a sizing cast for the target type.  A sizing
cast is a cast from that type to itself.  If one is found in the
<structname>pg_cast</structname> catalog, apply it to the expression before storing
into the destination column.  The implementation function for such a cast
always takes an extra parameter of type <type>integer</type>, which receives
the destination column's <structfield>atttypmod</structfield> value (typically its
declared length, although the interpretation of <structfield>atttypmod</structfield>
varies for different data types), and it may take a third <type>boolean</type>
parameter that says whether the cast is explicit or implicit.  The cast
function
is responsible for applying any length-dependent semantics such as size
checking or truncation.
</para>
</step>

</procedure>

<example>
<title><type>character</type> Storage Type Conversion</title>

<para>
For a target column declared as <type>character(20)</type> the following
statement shows that the stored value is sized correctly:

<screen>
CREATE TABLE vv (v character(20));
INSERT INTO vv SELECT 'abc' || 'def';
SELECT v, octet_length(v) FROM vv;

          v           | octet_length
----------------------+--------------
 abcdef               |           20
(1 row)
</screen>
</para>

<para>
What has really happened here is that the two unknown literals are resolved
to <type>text</type> by default, allowing the <literal>||</literal> operator
to be resolved as <type>text</type> concatenation.  Then the <type>text</type>
result of the operator is converted to <type>bpchar</type> (<quote>blank-padded
char</quote>, the internal name of the <type>character</type> data type) to match the target
column type.  (Since the conversion from <type>text</type> to
<type>bpchar</type> is binary-coercible, this conversion does
not insert any real function call.)  Finally, the sizing function
<literal>bpchar(bpchar, integer, boolean)</literal> is found in the system catalog
and applied to the operator's result and the stored column length.  This
type-specific function performs the required length check and addition of
padding spaces.
</para>
</example>
</sect1>

<sect1 id="typeconv-union-case">
<title><literal>UNION</literal>, <literal>CASE</literal>, and Related Constructs</title>

<indexterm zone="typeconv-union-case">
 <primary>UNION</primary>
 <secondary>determination of result type</secondary>
</indexterm>

<indexterm zone="typeconv-union-case">
 <primary>CASE</primary>
 <secondary>determination of result type</secondary>
</indexterm>

<indexterm zone="typeconv-union-case">
 <primary>ARRAY</primary>
 <secondary>determination of result type</secondary>
</indexterm>

<indexterm zone="typeconv-union-case">
 <primary>VALUES</primary>
 <secondary>determination of result type</secondary>
</indexterm>

<indexterm zone="typeconv-union-case">
 <primary>GREATEST</primary>
 <secondary>determination of result type</secondary>
</indexterm>

<indexterm zone="typeconv-union-case">
 <primary>LEAST</primary>
 <secondary>determination of result type</secondary>
</indexterm>

<para>
SQL <literal>UNION</literal> constructs must match up possibly dissimilar
types to become a single result set.  The resolution algorithm is
applied separately to each output column of a union query.  The
<literal>INTERSECT</literal> and <literal>EXCEPT</literal> constructs resolve
dissimilar types in the same way as <literal>UNION</literal>.
Some other constructs, including
<literal>CASE</literal>, <literal>ARRAY</literal>, <literal>VALUES</literal>,
and the <function>GREATEST</function> and <function>LEAST</function>
functions, use the identical
algorithm to match up their component expressions and select a result
data type.
</para>

<procedure>
<title>Type