1082 lines
35 KiB
Plaintext
1082 lines
35 KiB
Plaintext
<!-- doc/src/sgml/typeconv.sgml -->
|
|
|
|
<chapter id="typeconv">
|
|
<title>Type Conversion</title>
|
|
|
|
<indexterm zone="typeconv">
|
|
<primary>data type</primary>
|
|
<secondary>conversion</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
<acronym>SQL</acronym> statements can, intentionally or not, require
|
|
the mixing of different data types in the same expression.
|
|
<productname>PostgreSQL</productname> has extensive facilities for
|
|
evaluating mixed-type expressions.
|
|
</para>
|
|
|
|
<para>
|
|
In many cases a user does not need
|
|
to understand the details of the type conversion mechanism.
|
|
However, implicit conversions done by <productname>PostgreSQL</productname>
|
|
can affect the results of a query. When necessary, these results
|
|
can be tailored by using <emphasis>explicit</emphasis> type conversion.
|
|
</para>
|
|
|
|
<para>
|
|
This chapter introduces the <productname>PostgreSQL</productname>
|
|
type conversion mechanisms and conventions.
|
|
Refer to the relevant sections in <xref linkend="datatype"> and <xref linkend="functions">
|
|
for more information on specific data types and allowed functions and
|
|
operators.
|
|
</para>
|
|
|
|
<sect1 id="typeconv-overview">
|
|
<title>Overview</title>
|
|
|
|
<para>
|
|
<acronym>SQL</acronym> is a strongly typed language. That is, every data item
|
|
has an associated data type which determines its behavior and allowed usage.
|
|
<productname>PostgreSQL</productname> has an extensible type system that is
|
|
more general and flexible than other <acronym>SQL</acronym> implementations.
|
|
Hence, most type conversion behavior in <productname>PostgreSQL</productname>
|
|
is governed by general rules rather than by <foreignphrase>ad hoc</>
|
|
heuristics. This allows the use of mixed-type expressions even with
|
|
user-defined types.
|
|
</para>
|
|
|
|
<para>
|
|
The <productname>PostgreSQL</productname> scanner/parser divides lexical
|
|
elements into five fundamental categories: integers, non-integer numbers,
|
|
strings, identifiers, and key words. Constants of most non-numeric types are
|
|
first classified as strings. The <acronym>SQL</acronym> language definition
|
|
allows specifying type names with strings, and this mechanism can be used in
|
|
<productname>PostgreSQL</productname> to start the parser down the correct
|
|
path. For example, the query:
|
|
|
|
<screen>
|
|
SELECT text 'Origin' AS "label", point '(0,0)' AS "value";
|
|
|
|
label | value
|
|
--------+-------
|
|
Origin | (0,0)
|
|
(1 row)
|
|
</screen>
|
|
|
|
has two literal constants, of type <type>text</type> and <type>point</type>.
|
|
If a type is not specified for a string literal, then the placeholder type
|
|
<type>unknown</type> is assigned initially, to be resolved in later
|
|
stages as described below.
|
|
</para>
|
|
|
|
<para>
|
|
There are four fundamental <acronym>SQL</acronym> constructs requiring
|
|
distinct type conversion rules in the <productname>PostgreSQL</productname>
|
|
parser:
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term>
|
|
Function calls
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Much of the <productname>PostgreSQL</productname> type system is built around a
|
|
rich set of functions. Functions can have one or more arguments.
|
|
Since <productname>PostgreSQL</productname> permits function
|
|
overloading, the function name alone does not uniquely identify the function
|
|
to be called; the parser must select the right function based on the data
|
|
types of the supplied arguments.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term>
|
|
Operators
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
<productname>PostgreSQL</productname> allows expressions with
|
|
prefix and postfix unary (one-argument) operators,
|
|
as well as binary (two-argument) operators. Like functions, operators can
|
|
be overloaded, so the same problem of selecting the right operator
|
|
exists.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term>
|
|
Value Storage
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
<acronym>SQL</acronym> <command>INSERT</command> and <command>UPDATE</command> statements place the results of
|
|
expressions into a table. The expressions in the statement must be matched up
|
|
with, and perhaps converted to, the types of the target columns.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term>
|
|
<literal>UNION</literal>, <literal>CASE</literal>, and related constructs
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Since all query results from a unionized <command>SELECT</command> statement
|
|
must appear in a single set of columns, the types of the results of each
|
|
<command>SELECT</> clause must be matched up and converted to a uniform set.
|
|
Similarly, the result expressions of a <literal>CASE</> construct must be
|
|
converted to a common type so that the <literal>CASE</> expression as a whole
|
|
has a known output type. The same holds for <literal>ARRAY</> constructs,
|
|
and for the <function>GREATEST</> and <function>LEAST</> functions.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</para>
|
|
|
|
<para>
|
|
The system catalogs store information about which conversions, or
|
|
<firstterm>casts</firstterm>, exist between which data types, and how to
|
|
perform those conversions. Additional casts can be added by the user
|
|
with the <xref linkend="sql-createcast">
|
|
command. (This is usually
|
|
done in conjunction with defining new data types. The set of casts
|
|
between built-in types has been carefully crafted and is best not
|
|
altered.)
|
|
</para>
|
|
|
|
<indexterm>
|
|
<primary>data type</primary>
|
|
<secondary>category</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
An additional heuristic provided by the parser allows improved determination
|
|
of the proper casting behavior among groups of types that have implicit casts.
|
|
Data types are divided into several basic <firstterm>type
|
|
categories</firstterm>, including <type>boolean</type>, <type>numeric</type>,
|
|
<type>string</type>, <type>bitstring</type>, <type>datetime</type>,
|
|
<type>timespan</type>, <type>geometric</type>, <type>network</type>, and
|
|
user-defined. (For a list see <xref linkend="catalog-typcategory-table">;
|
|
but note it is also possible to create custom type categories.) Within each
|
|
category there can be one or more <firstterm>preferred types</firstterm>, which
|
|
are preferred when there is a choice of possible types. With careful selection
|
|
of preferred types and available implicit casts, it is possible to ensure that
|
|
ambiguous expressions (those with multiple candidate parsing solutions) can be
|
|
resolved in a useful way.
|
|
</para>
|
|
|
|
<para>
|
|
All type conversion rules are designed with several principles in mind:
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Implicit conversions should never have surprising or unpredictable outcomes.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
There should be no extra overhead in the parser or executor
|
|
if a query does not need implicit type conversion.
|
|
That is, if a query is well-formed and the types already match, then the query should execute
|
|
without spending extra time in the parser and without introducing unnecessary implicit conversion
|
|
calls in the query.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
Additionally, if a query usually requires an implicit conversion for a function, and
|
|
if then the user defines a new function with the correct argument types, the parser
|
|
should use this new function and no longer do implicit conversion to use the old function.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
</sect1>
|
|
|
|
<sect1 id="typeconv-oper">
|
|
<title>Operators</title>
|
|
|
|
<indexterm zone="typeconv-oper">
|
|
<primary>operator</primary>
|
|
<secondary>type resolution in an invocation</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
The specific operator that is referenced by an operator expression
|
|
is determined using the following procedure.
|
|
Note that this procedure is indirectly affected
|
|
by the precedence of the operators involved, since that will determine
|
|
which sub-expressions are taken to be the inputs of which operators.
|
|
See <xref linkend="sql-precedence"> for more information.
|
|
</para>
|
|
|
|
<procedure>
|
|
<title>Operator Type Resolution</title>
|
|
|
|
<step id="op-resol-select" performance="required">
|
|
<para>
|
|
Select the operators to be considered from the
|
|
<classname>pg_operator</classname> system catalog. If a non-schema-qualified
|
|
operator name was used (the usual case), the operators
|
|
considered are those with the matching name and argument count that are
|
|
visible in the current search path (see <xref linkend="ddl-schemas-path">).
|
|
If a qualified operator name was given, only operators in the specified
|
|
schema are considered.
|
|
</para>
|
|
|
|
<substeps>
|
|
<step performance="optional">
|
|
<para>
|
|
If the search path finds multiple operators with identical argument types,
|
|
only the one appearing earliest in the path is considered. Operators with
|
|
different argument types are considered on an equal footing regardless of
|
|
search path position.
|
|
</para>
|
|
</step>
|
|
</substeps>
|
|
</step>
|
|
|
|
<step id="op-resol-exact-match" performance="required">
|
|
<para>
|
|
Check for an operator accepting exactly the input argument types.
|
|
If one exists (there can be only one exact match in the set of
|
|
operators considered), use it.
|
|
</para>
|
|
|
|
<substeps>
|
|
<step id="op-resol-exact-unknown" performance="optional">
|
|
<para>
|
|
If one argument of a binary operator invocation is of the <type>unknown</type> type,
|
|
then assume it is the same type as the other argument for this check.
|
|
Invocations involving two <type>unknown</type> inputs, or a unary operator
|
|
with an <type>unknown</type> input, will never find a match at this step.
|
|
</para>
|
|
</step>
|
|
<step id="op-resol-exact-domain" performance="optional">
|
|
<para>
|
|
If one argument of a binary operator invocation is of the <type>unknown</type>
|
|
type and the other is of a domain type, next check to see if there is an
|
|
operator accepting exactly the domain's base type on both sides; if so, use it.
|
|
</para>
|
|
</step>
|
|
</substeps>
|
|
</step>
|
|
|
|
<step id="op-resol-best-match" performance="required">
|
|
<para>
|
|
Look for the best match.
|
|
</para>
|
|
<substeps>
|
|
<step performance="required">
|
|
<para>
|
|
Discard candidate operators 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-operator 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 id="op-resol-last-unknown" 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>
|
|
Some examples follow.
|
|
</para>
|
|
|
|
<example>
|
|
<title>Factorial Operator Type Resolution</title>
|
|
|
|
<para>
|
|
There is only one factorial operator (postfix <literal>!</>)
|
|
defined in the standard catalog, and it takes an argument of type
|
|
<type>bigint</type>.
|
|
The scanner assigns an initial type of <type>integer</type> to the argument
|
|
in this query expression:
|
|
<screen>
|
|
SELECT 40 ! AS "40 factorial";
|
|
|
|
40 factorial
|
|
--------------------------------------------------
|
|
815915283247897734345611269596115894272000000000
|
|
(1 row)
|
|
</screen>
|
|
|
|
So the parser does a type conversion on the operand and the query
|
|
is equivalent to:
|
|
|
|
<screen>
|
|
SELECT CAST(40 AS bigint) ! AS "40 factorial";
|
|
</screen>
|
|
</para>
|
|
</example>
|
|
|
|
<example>
|
|
<title>String Concatenation Operator Type Resolution</title>
|
|
|
|
<para>
|
|
A string-like syntax is used for working with string types and for
|
|
working with complex extension types.
|
|
Strings with unspecified type are matched with likely operator candidates.
|
|
</para>
|
|
|
|
<para>
|
|
An example with one unspecified argument:
|
|
<screen>
|
|
SELECT text 'abc' || 'def' AS "text and unknown";
|
|
|
|
text and unknown
|
|
------------------
|
|
abcdef
|
|
(1 row)
|
|
</screen>
|
|
</para>
|
|
|
|
<para>
|
|
In this case the parser looks to see if there is an operator taking <type>text</type>
|
|
for both arguments. Since there is, it assumes that the second argument should
|
|
be interpreted as type <type>text</type>.
|
|
</para>
|
|
|
|
<para>
|
|
Here is a concatenation of two values of unspecified types:
|
|
<screen>
|
|
SELECT 'abc' || 'def' AS "unspecified";
|
|
|
|
unspecified
|
|
-------------
|
|
abcdef
|
|
(1 row)
|
|
</screen>
|
|
</para>
|
|
|
|
<para>
|
|
In this case there is no initial hint for which type to use, since no types
|
|
are specified in the query. So, the parser looks for all candidate operators
|
|
and finds that there are candidates accepting both string-category and
|
|
bit-string-category inputs. Since string category is preferred when available,
|
|
that category is selected, and then the
|
|
preferred type for strings, <type>text</type>, is used as the specific
|
|
type to resolve the unknown-type literals as.
|
|
</para>
|
|
</example>
|
|
|
|
<example>
|
|
<title>Absolute-Value and Negation Operator Type Resolution</title>
|
|
|
|
<para>
|
|
The <productname>PostgreSQL</productname> operator catalog has several
|
|
entries for the prefix operator <literal>@</>, all of which implement
|
|
absolute-value operations for various numeric data types. One of these
|
|
entries is for type <type>float8</type>, which is the preferred type in
|
|
the numeric category. Therefore, <productname>PostgreSQL</productname>
|
|
will use that entry when faced with an <type>unknown</> input:
|
|
<screen>
|
|
SELECT @ '-4.5' AS "abs";
|
|
abs
|
|
-----
|
|
4.5
|
|
(1 row)
|
|
</screen>
|
|
Here the system has implicitly resolved the unknown-type literal as type
|
|
<type>float8</type> before applying the chosen operator. We can verify that
|
|
<type>float8</type> and not some other type was used:
|
|
<screen>
|
|
SELECT @ '-4.5e500' AS "abs";
|
|
|
|
ERROR: "-4.5e500" is out of range for type double precision
|
|
</screen>
|
|
</para>
|
|
|
|
<para>
|
|
On the other hand, the prefix operator <literal>~</> (bitwise negation)
|
|
is defined only for integer data types, not for <type>float8</type>. So, if we
|
|
try a similar case with <literal>~</>, we get:
|
|
<screen>
|
|
SELECT ~ '20' AS "negation";
|
|
|
|
ERROR: operator is not unique: ~ "unknown"
|
|
HINT: Could not choose a best candidate operator. You might need to add
|
|
explicit type casts.
|
|
</screen>
|
|
This happens because the system cannot decide which of the several
|
|
possible <literal>~</> operators should be preferred. We can help
|
|
it out with an explicit cast:
|
|
<screen>
|
|
SELECT ~ CAST('20' AS int8) AS "negation";
|
|
|
|
negation
|
|
----------
|
|
-21
|
|
(1 row)
|
|
</screen>
|
|
</para>
|
|
</example>
|
|
|
|
<example>
|
|
<title>Array Inclusion Operator Type Resolution</title>
|
|
|
|
<para>
|
|
Here is another example of resolving an operator with one known and one
|
|
unknown input:
|
|
<screen>
|
|
SELECT array[1,2] <@ '{1,2,3}' as "is subset";
|
|
|
|
is subset
|
|
-----------
|
|
t
|
|
(1 row)
|
|
</screen>
|
|
The <productname>PostgreSQL</productname> operator catalog has several
|
|
entries for the infix operator <literal><@</>, but the only two that
|
|
could possibly accept an integer array on the left-hand side are
|
|
array inclusion (<type>anyarray</> <literal><@</> <type>anyarray</>)
|
|
and range inclusion (<type>anyelement</> <literal><@</> <type>anyrange</>).
|
|
Since none of these polymorphic pseudo-types (see <xref
|
|
linkend="datatype-pseudo">) are considered preferred, the parser cannot
|
|
resolve the ambiguity on that basis.
|
|
However, <xref linkend="op-resol-last-unknown"> tells
|
|
it to assume that the unknown-type literal is of the same type as the other
|
|
input, that is, integer array. Now only one of the two operators can match,
|
|
so array inclusion is selected. (Had range inclusion been selected, we would
|
|
have gotten an error, because the string does not have the right format to be
|
|
a range literal.)
|
|
</para>
|
|
</example>
|
|
|
|
<example>
|
|
<title>Custom Operator on a Domain Type</title>
|
|
|
|
<para>
|
|
Users sometimes try to declare operators applying just to a domain type.
|
|
This is possible but is not nearly as useful as it might seem, because the
|
|
operator resolution rules are designed to select operators applying to the
|
|
domain's base type. As an example consider
|
|
<screen>
|
|
CREATE DOMAIN mytext AS text CHECK(...);
|
|
CREATE FUNCTION mytext_eq_text (mytext, text) RETURNS boolean AS ...;
|
|
CREATE OPERATOR = (procedure=mytext_eq_text, leftarg=mytext, rightarg=text);
|
|
CREATE TABLE mytable (val mytext);
|
|
|
|
SELECT * FROM mytable WHERE val = 'foo';
|
|
</screen>
|
|
This query will not use the custom operator. The parser will first see if
|
|
there is a <type>mytext</> <literal>=</> <type>mytext</> operator
|
|
(<xref linkend="op-resol-exact-unknown">), which there is not;
|
|
then it will consider the domain's base type <type>text</>, and see if
|
|
there is a <type>text</> <literal>=</> <type>text</> operator
|
|
(<xref linkend="op-resol-exact-domain">), which there is;
|
|
so it resolves the <type>unknown</>-type literal as <type>text</> and
|
|
uses the <type>text</> <literal>=</> <type>text</> operator.
|
|
The only way to get the custom operator to be used is to explicitly cast
|
|
the literal:
|
|
<screen>
|
|
SELECT * FROM mytable WHERE val = text 'foo';
|
|
</screen>
|
|
so that the <type>mytext</> <literal>=</> <type>text</> operator is found
|
|
immediately according to the exact-match rule. If the best-match rules
|
|
are reached, they actively discriminate against operators on domain types.
|
|
If they did not, such an operator would create too many ambiguous-operator
|
|
failures, because the casting rules always consider a domain as castable
|
|
to or from its base type, and so the domain operator would be considered
|
|
usable in all the same cases as a similarly-named operator on the base type.
|
|
</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 that is referenced by a function call
|
|
is determined using the following procedure.
|
|
</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 a non-schema-qualified
|
|
function name was used, the functions
|
|
considered are those with the matching 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. Functions of
|
|
different argument types are considered on an equal footing regardless of
|
|
search path position.
|
|
</para>
|
|
</step>
|
|
<step performance="optional">
|
|
<para>
|
|
If a function is declared with a <literal>VARIADIC</> array parameter, and
|
|
the call does not use the <literal>VARIADIC</> keyword, then the function
|
|
is treated as if the array parameter were replaced by one or more occurrences
|
|
of its element type, as needed to match the call. After such expansion the
|
|
function might have effective argument types identical to some non-variadic
|
|
function. In that case the function appearing earlier in the search path is
|
|
used, or if the two functions are in the same schema, the non-variadic one is
|
|
preferred.
|
|
</para>
|
|
</step>
|
|
<step performance="optional">
|
|
<para>
|
|
Functions that have default values for parameters are considered to match any
|
|
call that omits zero or more of the defaultable parameter positions. If more
|
|
than one such function matches a call, the one appearing earliest in the
|
|
search path is used. If there are two or more such functions in the same
|
|
schema with identical parameter types in the non-defaulted positions (which is
|
|
possible if they have different sets of defaultable parameters), the system
|
|
will not be able to determine which to prefer, and so an <quote>ambiguous
|
|
function call</> error will result if no better match to the call can be
|
|
found.
|
|
</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 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</> 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</> 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 that takes two
|
|
arguments; it takes a first argument of type <type>numeric</type> and
|
|
a second argument of type <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 therefore might 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
|
|
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);
|
|
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.
|
|
</screen>
|
|
|
|
This does not work because <type>integer</> does not have an implicit cast
|
|
to <type>text</>. An explicit cast will work, however:
|
|
<screen>
|
|
SELECT substr(CAST (1234 AS text), 3);
|
|
|
|
substr
|
|
--------
|
|
34
|
|
(1 row)
|
|
</screen>
|
|
</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 is possible
|
|
if an <firstterm>assignment cast</> between the two types is registered in the
|
|
<structname>pg_cast</> catalog (see <xref 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</> 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</> value (typically its
|
|
declared length, although the interpretation of <structfield>atttypmod</>
|
|
varies for different data types), and it may take a third <type>boolean</>
|
|
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</>, 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)</> 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</> 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</>, <literal>ARRAY</>, <literal>VALUES</>,
|
|
<function>GREATEST</> and <function>LEAST</> constructs use the identical
|
|
algorithm to match up their component expressions and select a result
|
|
data type.
|
|
</para>
|
|
|
|
<procedure>
|
|
<title>Type Resolution for <literal>UNION</literal>, <literal>CASE</literal>,
|
|
and Related Constructs</title>
|
|
|
|
<step performance="required">
|
|
<para>
|
|
If all inputs are of the same type, and it is not <type>unknown</type>,
|
|
resolve as that type.
|
|
</para>
|
|
</step>
|
|
|
|
<step performance="required">
|
|
<para>
|
|
If any input is of a domain type, treat it as being of the
|
|
domain's base type for all subsequent steps.
|
|
<footnote>
|
|
<para>
|
|
Somewhat like the treatment of domain inputs for operators and
|
|
functions, this behavior allows a domain type to be preserved through
|
|
a <literal>UNION</> or similar construct, so long as the user is
|
|
careful to ensure that all inputs are implicitly or explicitly of that
|
|
exact type. Otherwise the domain's base type will be preferred.
|
|
</para>
|
|
</footnote>
|
|
</para>
|
|
</step>
|
|
|
|
<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, <type>unknown</type> inputs are ignored.
|
|
</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, if there is one.
|
|
</para>
|
|
</step>
|
|
|
|
<step performance="required">
|
|
<para>
|
|
Otherwise, choose the last non-unknown input type that allows all the
|
|
preceding non-unknown inputs to be implicitly converted to it. (There
|
|
always is such a type, since at least the first type in the list must
|
|
satisfy this condition.)
|
|
</para>
|
|
</step>
|
|
|
|
<step performance="required">
|
|
<para>
|
|
Convert all inputs to the selected type. Fail if there is not a
|
|
conversion from a given input 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 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</> 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>
|