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>PostgreSQL 8.3.6 Documentation</TH
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>Chapter 10. Type Conversion</TD
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><H1
CLASS="SECT1"
><A
NAME="TYPECONV-FUNC"
>10.3. Functions</A
></H1
><A
NAME="AEN17297"
></A
><P
>   The specific function to be used in a function invocation is determined
   according to the following steps.
  </P
><DIV
CLASS="PROCEDURE"
><P
><B
>Function Type Resolution</B
></P
><OL
TYPE="1"
><LI
CLASS="STEP"
><P
>Select the functions to be considered from the
<CODE
CLASS="CLASSNAME"
>pg_proc</CODE
> system catalog.  If an unqualified
function name was used, the functions
considered are those of the right name and argument count that are
visible in the current search path (see <A
HREF="ddl-schemas.html#DDL-SCHEMAS-PATH"
>Section 5.7.3</A
>).
If a qualified function name was given, only functions in the specified
schema are considered.</P
><OL
CLASS="SUBSTEPS"
TYPE="a"
><LI
CLASS="STEP"
><P
>If the search path finds multiple functions of identical argument types,
only the one appearing earliest in the path is considered.  But functions of
different argument types are considered on an equal footing regardless of
search path position.</P
></LI
></OL
></LI
><LI
CLASS="STEP"
><P
>Check for a function accepting exactly the input argument types.
If one exists (there can be only one exact match in the set of
functions considered), use it.
(Cases involving <TT
CLASS="TYPE"
>unknown</TT
> will never find a match at
this step.)</P
></LI
><LI
CLASS="STEP"
><P
>If no exact match is found, see whether 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-compatible with 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 <TT
CLASS="LITERAL"
>CAST</TT
> specification.
  <A
NAME="AEN17316"
HREF="#FTN.AEN17316"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></P
></LI
><LI
CLASS="STEP"
><P
>Look for the best match.</P
><OL
CLASS="SUBSTEPS"
TYPE="a"
><LI
CLASS="STEP"
><P
>Discard candidate functions for which the input types do not match
and cannot be converted (using an implicit conversion) to match.
<TT
CLASS="TYPE"
>unknown</TT
> literals are
assumed to be convertible to anything for this purpose.  If only one
candidate remains, use it; else continue to the next step.</P
></LI
><LI
CLASS="STEP"
><P
>Run through all candidates and keep those with the most exact matches
on input types.  (Domains are considered the same as their base type
for this purpose.)  Keep all candidates if none have any exact matches.
If only one candidate remains, use it; else continue to the next step.</P
></LI
><LI
CLASS="STEP"
><P
>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.</P
></LI
><LI
CLASS="STEP"
><P
>If any input arguments are <TT
CLASS="TYPE"
>unknown</TT
>, check the type categories
accepted 
at those argument positions by the remaining candidates.  At each position,
select the <TT
CLASS="TYPE"
>string</TT
> category if any candidate accepts that category.
(This bias towards string
is appropriate since an unknown-type literal does look 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 at a given argument
position, discard candidates that accept non-preferred types for that
argument.</P
></LI
><LI
CLASS="STEP"
><P
>If only one candidate remains, use it.  If no candidate or more than one
candidate remains,
then fail.</P
></LI
></OL
></LI
></OL
></DIV
><P
>Note that the <SPAN
CLASS="QUOTE"
>"best match"</SPAN
> rules are identical for operator and
function type resolution.
Some examples follow.</P
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN17337"
></A
><P
><B
>Example 10-4. Rounding Function Argument Type Resolution</B
></P
><P
>There is only one <CODE
CLASS="FUNCTION"
>round</CODE
> function with two
arguments.  (The first is <TT
CLASS="TYPE"
>numeric</TT
>, the second is
<TT
CLASS="TYPE"
>integer</TT
>.)  So the following query automatically converts
the first argument of type <TT
CLASS="TYPE"
>integer</TT
> to
<TT
CLASS="TYPE"
>numeric</TT
>:

</P><PRE
CLASS="SCREEN"
>SELECT round(4, 4);

 round
--------
 4.0000
(1 row)</PRE
><P>

That query is actually transformed by the parser to
</P><PRE
CLASS="SCREEN"
>SELECT round(CAST (4 AS numeric), 4);</PRE
><P></P
><P
>Since numeric constants with decimal points are initially assigned the
type <TT
CLASS="TYPE"
>numeric</TT
>, the following query will require no type
conversion and might therefore be slightly more efficient:
</P><PRE
CLASS="SCREEN"
>SELECT round(4.0, 4);</PRE
><P></P
></DIV
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN17350"
></A
><P
><B
>Example 10-5. Substring Function Type Resolution</B
></P
><P
>There are several <CODE
CLASS="FUNCTION"
>substr</CODE
> functions, one of which
takes types <TT
CLASS="TYPE"
>text</TT
> and <TT
CLASS="TYPE"
>integer</TT
>.  If called
with a string constant of unspecified type, the system chooses the
candidate function that accepts an argument of the preferred category
<TT
CLASS="LITERAL"
>string</TT
> (namely of type <TT
CLASS="TYPE"
>text</TT
>).

</P><PRE
CLASS="SCREEN"
>SELECT substr('1234', 3);

 substr
--------
     34
(1 row)</PRE
><P></P
><P
>If the string is declared to be of type <TT
CLASS="TYPE"
>varchar</TT
>, as might be the case
if it comes from a table, then the parser will try to convert it to become <TT
CLASS="TYPE"
>text</TT
>:
</P><PRE
CLASS="SCREEN"
>SELECT substr(varchar '1234', 3);

 substr
--------
     34
(1 row)</PRE
><P>

This is transformed by the parser to effectively become
</P><PRE
CLASS="SCREEN"
>SELECT substr(CAST (varchar '1234' AS text), 3);</PRE
><P></P
><P
></P><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>The parser learns from the <TT
CLASS="STRUCTNAME"
>pg_cast</TT
> catalog that
<TT
CLASS="TYPE"
>text</TT
> and <TT
CLASS="TYPE"
>varchar</TT
>
are binary-compatible, meaning that one can be passed to a function that
accepts the other without doing any physical conversion.  Therefore, no
type conversion call is really inserted in this case.</P
></BLOCKQUOTE
></DIV
><P></P
><P
>And, if the function is called with an argument of type <TT
CLASS="TYPE"
>integer</TT
>,
the parser will try to convert that to <TT
CLASS="TYPE"
>text</TT
>:
</P><PRE
CLASS="SCREEN"
>SELECT substr(1234, 3);
ERROR:  function substr(integer, integer) does not exist
HINT:  No function matches the given name and argument types. You might need
to add explicit type casts.</PRE
><P>

This does not work because <TT
CLASS="TYPE"
>integer</TT
> does not have an implicit cast
to <TT
CLASS="TYPE"
>text</TT
>.  An explicit cast will work, however:
</P><PRE
CLASS="SCREEN"
>SELECT substr(CAST (1234 AS text), 3);

 substr
--------
     34
(1 row)</PRE
><P></P
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
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><A
NAME="FTN.AEN17316"
HREF="typeconv-func.html#AEN17316"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
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><P
>    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
    <A
HREF="sql-createcast.html"
><I
>CREATE CAST</I
></A
>
    for additional commentary.
   </P
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