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Add support for user-defined I/O conversion casts.

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This commit is contained in:
Heikki Linnakangas
2008-10-31 08:39:22 +00:00
parent 34e37d58ed
commit 092bc49653
14 changed files with 396 additions and 269 deletions
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38
doc/src/sgml/ref/create_cast.sgml
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@@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/create_cast.sgml,v 1.29 2008/07/30 21:23:17 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/create_cast.sgml,v 1.30 2008/10/31 08:39:20 heikki Exp $ -->
<refentry id="SQL-CREATECAST">
<refmeta>
@@ -24,6 +24,10 @@ CREATE CAST (<replaceable>sourcetype</replaceable> AS <replaceable>targettype</r
CREATE CAST (<replaceable>sourcetype</replaceable> AS <replaceable>targettype</replaceable>)
WITHOUT FUNCTION
[ AS ASSIGNMENT | AS IMPLICIT ]
CREATE CAST (<replaceable>sourcetype</replaceable> AS <replaceable>targettype</replaceable>)
WITH INOUT
[ AS ASSIGNMENT | AS IMPLICIT ]
</synopsis>
</refsynopsisdiv>
@@ -58,6 +62,13 @@ SELECT CAST(42 AS float8);
binary compatible.)
</para>
<para>
You can define a cast as an <firstterm>I/O conversion cast</> using
the <literal>WITH INOUT</literal> syntax. An I/O conversion cast is
performed by invoking the output function of the source data type, and
passing the result to the input function of the target data type.
</para>
<para>
By default, a cast can be invoked only by an explicit cast request,
that is an explicit <literal>CAST(<replaceable>x</> AS
@@ -199,6 +210,18 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
</listitem>
</varlistentry>
<varlistentry>
<term><literal>WITH INOUT</literal></term>
<listitem>
<para>
Indicates that the cast is an I/O conversion cast, performed by
invoking the output function of the source data type, and passing the
result to the input function of the target data type.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>AS ASSIGNMENT</literal></term>
@@ -284,15 +307,12 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
It is normally not necessary to create casts between user-defined types
and the standard string types (<type>text</>, <type>varchar</>, and
<type>char(<replaceable>n</>)</type>, as well as user-defined types that
are defined to be in the string category). <productname>PostgreSQL</> will
automatically handle a cast to a string type by invoking the other
type's output function, or conversely handle a cast from a string type
by invoking the other type's input function. These
automatically-provided casts are known as <firstterm>I/O conversion
casts</>. I/O conversion casts to string types are treated as
assignment casts, while I/O conversion casts from string types are
are defined to be in the string category). <productname>PostgreSQL</>
provides automatic I/O conversion casts for that. The automatic casts to
string types are treated as assignment casts, while the automatic casts
from string types are
explicit-only. You can override this behavior by declaring your own
cast to replace an I/O conversion cast, but usually the only reason to
cast to replace an automatic cast, but usually the only reason to
do so is if you want the conversion to be more easily invokable than the
standard assignment-only or explicit-only setting. Another possible
reason is that you want the conversion to behave differently from the