typeconv.sgml

PostgreSQL7.4.6 for Linux

 2432902008176640000
(1 row)
</screen>
</para>
</example>

</sect1>

<sect1 id="typeconv-func">
<title>Functions</title>

<indexterm zone="typeconv-func">
 <primary>function</primary>
 <secondary>type resolution in an invocation</secondary>
</indexterm>

  <para>
   The specific function to be used in a function invocation is determined
   according to the following steps.
  </para>

<procedure>
<title>Function Type Resolution</title>

<step performance="required">
<para>
Select the functions to be considered from the
<classname>pg_proc</classname> 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 <xref linkend="ddl-schemas-path">).
If a qualified function name was given, only functions in the specified
schema are considered.
</para>

<substeps>
<step performance="optional">
<para>
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.
</para>
</step>
</substeps>
</step>

<step performance="required">
<para>
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 <type>unknown</type> will never find a match at
this step.)
</para>
</step>

<step performance="required">
<para>
If no exact match is found, see whether the function call appears
to be a trivial 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.  When these conditions are met, the function argument is converted
to the named data type without any actual function call.
</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>
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.
</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 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.
</para>
</step>
<step performance="required">
<para>
If only one candidate remains, use it.  If no candidate or more than one
candidate remains,
then fail.
</para>
</step>
</substeps>
</step>
</procedure>

<para>
Note that the <quote>best match</> rules are identical for operator and
function type resolution.
Some examples follow.
</para>

<example>
<title>Rounding Function Argument Type Resolution</title>

<para>
There is only one <function>round</function> function with two
arguments.  (The first is <type>numeric</type>, the second is
<type>integer</type>.)  So the following query automatically converts
the first argument of type <type>integer</type> to
<type>numeric</type>:

<screen>
SELECT round(4, 4);

 round
--------
 4.0000
(1 row)
</screen>

That query is actually transformed by the parser to
<screen>
SELECT round(CAST (4 AS numeric), 4);
</screen>
</para>

<para>
Since numeric constants with decimal points are initially assigned the
type <type>numeric</type>, the following query will require no type
conversion and may therefore be slightly more efficient:
<screen>
SELECT round(4.0, 4);
</screen>
</para>
</example>

<example>
<title>Substring Function Type Resolution</title>

<para>
There are several <function>substr</function> functions, one of which
takes types <type>text</type> and <type>integer</type>.  If called
with a string constant of unspecified type, the system chooses the
candidate function that accepts an argument of the preferred category
<literal>string</literal> (namely of type <type>text</type>).

<screen>
SELECT substr('1234', 3);

 substr
--------
     34
(1 row)
</screen>
</para>

<para>
If the string is declared to be of type <type>varchar</type>, as might be the case
if it comes from a table, then the parser will try to convert it to become <type>text</type>:
<screen>
SELECT substr(varchar '1234', 3);

 substr
--------
     34
(1 row)
</screen>

This is transformed by the parser to effectively become
<screen>
SELECT substr(CAST (varchar '1234' AS text), 3);
</screen>
</para>
<para>
<note>
<para>
The parser learns from the <structname>pg_cast</> catalog that
<type>text</type> and <type>varchar</type>
are binary-compatible, meaning that one can be passed to a function that
accepts the other without doing any physical conversion.  Therefore, no
explicit type conversion call is really inserted in this case.
</para>
</note>
</para>

<para>
And, if the function is called with an argument of type <type>integer</type>, the parser will
try to convert that to <type>text</type>:
<screen>
SELECT substr(1234, 3);

 substr
--------
     34
(1 row)
</screen>

This actually executes as
<screen>
SELECT substr(CAST (1234 AS text), 3);
</screen>
This automatic transformation can succeed because there is an
implicitly invocable cast from <type>integer</type> to
<type>text</type>.
</para>
</example>

</sect1>

<sect1 id="typeconv-query">
<title>Value Storage</title>

  <para>
   Values to be inserted into a table are converted to the destination
   column's data type according to the
   following steps.
  </para>

<procedure>
<title>Value Storage Type Conversion</title>

<step performance="required">
<para>
Check for an exact match with the target.
</para>
</step>

<step performance="required">
<para>
Otherwise, try to convert the expression to the target type.  This will succeed
if there is a registered cast between the two types.
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>
If the target is a fixed-length type (e.g., <type>char</type> or <type>varchar</type>
declared with a length) then try to find a sizing function for the target
type.  A sizing function is a function of the same name as the type,
taking two arguments of which the first is that type and the second is of type
<type>integer</type>, and returning the same type.  If one is found, it is applied,
passing the column's declared length as the second parameter.
</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
ensures that the stored value is sized correctly:

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

          v           | 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</>, the internal name of the <type>character</type> data type) to match the target
column type.  (Since the types <type>text</type> and
<type>bpchar</type> are binary-compatible, this conversion does
not insert any real function call.)  Finally, the sizing function
<literal>bpchar(bpchar, integer)</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 <literal>ARRAY</literal> 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>

<para>
SQL <literal>UNION</> 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</> and <literal>EXCEPT</> constructs resolve
dissimilar types in the same way as <literal>UNION</>.  The
<literal>CASE</> and <literal>ARRAY</> constructs use the identical
algorithm to match up their component expressions and select a result
data type.
</para>

<procedure>
<title><literal>UNION</literal>, <literal>CASE</literal>, and
<literal>ARRAY</literal> Type Resolution</title>

<step performance="required">
<para>
If all inputs are of type <type>unknown</type>, resolve as type
<type>text</type> (the preferred type of the string category).
Otherwise, ignore the <type>unknown</type> inputs while choosing the result type.
</para>
</step>

<step performance="required">
<para>
If the non-unknown inputs are not all of the same type category, fail.
</para>
</step>

<step performance="required">
<para>
Choose the first non-unknown input type which is a preferred type in
that category or allows all the non-unknown inputs to be implicitly
converted to it.
</para>
</step>

<step performance="required">
<para>
Convert all inputs to the selected 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 as 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</> is of type <type>numeric</>,
and the <type>integer</type> value <literal>1</> can be cast implicitly to
<type>numeric</>, 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</> cannot be implicitly cast to <type>integer</>,
but <type>integer</> can be implicitly cast to <type>real</>, the union
result type is resolved as <type>real</>.
</para>
</example>

</sect1>
</chapter>

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