that permits untrusted users to create objects.  In such situations,
cast arguments to force an exact match.  (Cases involving <type>unknown</type>
will never find a match at this step.)
</para>
</step>

<step performance="required">
<para>
If no exact match is found, see if the function call appears
to be a special type conversion request.  This happens if the function call
has just one argument and the function name is the same as the (internal)
name of some data type.  Furthermore, the function argument must be either
an unknown-type literal, or a type that is binary-coercible to the named
data type, or a type that could be converted to the named data type by
applying that type's I/O functions (that is, the conversion is either to or
from one of the standard string types).  When these conditions are met,
the function call is treated as a form of <literal>CAST</literal> specification.
  <footnote>
   <para>
    The reason for this step is to support function-style cast specifications
    in cases where there is not an actual cast function.  If there is a cast
    function, it is conventionally named after its output type, and so there
    is no need to have a special case.  See
    <xref linkend="sql-createcast"/>
    for additional commentary.
   </para>
  </footnote>
</para>
</step>
<step performance="required">
<para>
Look for the best match.
</para>
<substeps>
<step performance="required">
<para>
Discard candidate functions for which the input types do not match
and cannot be converted (using an implicit conversion) to match.
<type>unknown</type> literals are
assumed to be convertible to anything for this purpose.  If only one
candidate remains, use it; else continue to the next step.
</para>
</step>
<step performance="required">
<para>
If any input argument is of a domain type, treat it as being of the
domain's base type for all subsequent steps.  This ensures that domains
act like their base types for purposes of ambiguous-function resolution.
</para>
</step>
<step performance="required">
<para>
Run through all candidates and keep those with the most exact matches
on input types.  Keep all candidates if none have exact matches.
If only one candidate remains, use it; else continue to the next step.
</para>
</step>
<step performance="required">
<para>
Run through all candidates and keep those that accept preferred types (of the
input data type's type category) at the most positions where type conversion
will be required.
Keep all candidates if none accept preferred types.
If only one candidate remains, use it; else continue to the next step.
</para>
</step>
<step performance="required">
<para>
If any input arguments are <type>unknown</type>, check the type categories
accepted
at those argument positions by the remaining candidates.  At each position,
select the <type>string</type> category if any candidate accepts that category.
(This bias towards string
is appropriate since an unknown-type literal looks like a string.)
Otherwise, if all the remaining candidates accept the same type category,
select that category; otherwise fail because
the correct choice cannot be deduced without more clues.
Now discard candidates that do not accept the selected type category.
Furthermore, if any candidate accepts a preferred type in that category,
discard candidates that accept non-preferred types for that argument.
Keep all candidates if none survive these tests.
If only one candidate remains, use it; else continue to the next step.
</para>
</step>
<step performance="required">
<para>
If there are both <type>unknown</type> and known-type arguments, and all
the known-type arguments have the same type, assume that the
<type>unknown</type> arguments are also of that type, and check which
candidates can accept that type at the <type>unknown</type>-argument
positions.  If exactly one candidate passes this test, use it.
Otherwise, fail.
</para>
</step>
</substeps>
</step>
</procedure>

<para>
Note that the <quote>best match</quote> rules are