390 lines
12 KiB
Plaintext
390 lines
12 KiB
Plaintext
<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_function.sgml,v 1.15 2000/07/22 04:30:27 momjian Exp $
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Postgres documentation
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-->
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<refentry id="SQL-CREATEFUNCTION">
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<refmeta>
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<refentrytitle id="sql-createfunction-title">
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CREATE FUNCTION
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</refentrytitle>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>
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CREATE FUNCTION
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</refname>
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<refpurpose>
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Defines a new function
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</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<refsynopsisdivinfo>
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<date>2000-03-25</date>
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</refsynopsisdivinfo>
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<synopsis>
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CREATE FUNCTION <replaceable class="parameter">name</replaceable> ( [ <replaceable class="parameter">ftype</replaceable> [, ...] ] )
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RETURNS <replaceable class="parameter">rtype</replaceable>
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AS <replaceable class="parameter">definition</replaceable>
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LANGUAGE '<replaceable class="parameter">langname</replaceable>'
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[ WITH ( <replaceable class="parameter">attribute</replaceable> [, ...] ) ]
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CREATE FUNCTION <replaceable class="parameter">name</replaceable> ( [ <replaceable class="parameter">ftype</replaceable> [, ...] ] )
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RETURNS <replaceable class="parameter">rtype</replaceable>
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AS <replaceable class="parameter">obj_file</replaceable> , <replaceable class="parameter">link_symbol</replaceable>
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LANGUAGE 'C'
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[ WITH ( <replaceable class="parameter">attribute</replaceable> [, ...] ) ]
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</synopsis>
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<refsect2 id="R2-SQL-CREATEFUNCTION-1">
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<refsect2info>
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<date>2000-03-25</date>
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</refsect2info>
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<title>
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Inputs
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</title>
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<para>
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<variablelist>
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<varlistentry>
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<term><replaceable class="parameter">name</replaceable></term>
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<listitem>
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<para>
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The name of a function to create.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">ftype</replaceable></term>
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<listitem>
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<para>
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The data type of function arguments.
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The input types may be base or complex types, or
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<firstterm>opaque</firstterm>.
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<literal>opaque</literal> indicates that the function
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accepts arguments of an invalid type such as <type>char *</type>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">rtype</replaceable></term>
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<listitem>
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<para>
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The return data type.
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The output type may be specified as a base type, complex type,
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<literal>setof <replaceable class="parameter">type</replaceable></literal>,
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or <literal>opaque</literal>.
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The <literal>setof</literal>
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modifier indicates that the function will return a set of items,
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rather than a single item.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">attribute</replaceable></term>
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<listitem>
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<para>
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An optional piece of information about the function, used for
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optimization. The only attribute currently supported is
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<literal>iscachable</literal>.
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<literal>iscachable</literal> indicates that the function always
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returns the same result when given the same input values (i.e.,
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it does not do database lookups or otherwise use information not
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directly present in its parameter list). The optimizer uses
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<literal>iscachable</literal> to know whether it is safe to
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pre-evaluate a call of the function.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">definition</replaceable></term>
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<listitem>
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<para>
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A string defining the function; the meaning depends on the language.
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It may be an internal function name, the path to an object file,
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an SQL query, or text in a procedural language.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">obj_file</replaceable> , <replaceable class="parameter">link_symbol</replaceable></term>
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<listitem>
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<para>
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This form of the <command>AS</command> clause is used for
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dynamically-linked, C language functions when the function name in
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the C language source code is not the same as the name of the SQL
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function. The string <replaceable
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class="parameter">obj_file</replaceable> is the name of the file
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containing the dynamically loadable object, and <replaceable
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class="parameter">link_symbol</replaceable>, is the object's link
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symbol which is the same as the name of the function in the C
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language source code.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">langname</replaceable></term>
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<listitem>
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<para>
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may be '<literal>C</literal>', '<literal>sql</literal>',
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'<literal>internal</literal>'
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or '<replaceable class="parameter">plname</replaceable>',
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where '<replaceable class="parameter">plname</replaceable>'
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is the name of a created procedural language. See
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<xref linkend="sql-createlanguage-title" endterm="sql-createlanguage-title">
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for details.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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<refsect2 id="R2-SQL-CREATEFUNCTION-2">
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<refsect2info>
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<date>2000-03-25</date>
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</refsect2info>
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<title>
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Outputs
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</title>
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<para>
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<variablelist>
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<varlistentry>
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<term><computeroutput>
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CREATE
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</computeroutput></term>
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<listitem>
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<para>
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This is returned if the command completes successfully.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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</refsynopsisdiv>
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<refsect1 id="R1-SQL-CREATEFUNCTION-1">
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<refsect1info>
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<date>2000-03-25</date>
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</refsect1info>
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<title>
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Description
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</title>
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<para>
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<command>CREATE FUNCTION</command> allows a
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<productname>Postgres</productname> user
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to register a function
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with a database. Subsequently, this user is considered the
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owner of the function.
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</para>
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<refsect2 id="R2-SQL-CREATEFUNCTION-3">
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<refsect2info>
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<date>2000-03-25</date>
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</refsect2info>
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<title>
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Notes
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</title>
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<para>
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Refer to the chapter in the
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<citetitle>PostgreSQL Programmer's Guide</citetitle>
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on the topic of extending
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<productname>Postgres</productname> via functions
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for further information on writing external functions.
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</para>
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<para>
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Use <command>DROP FUNCTION</command>
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to remove user-defined functions.
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</para>
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<para>
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<productname>Postgres</productname> allows function "overloading";
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that is, the same name can be used for several different functions
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so long as they have distinct argument types. This facility must
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be used with caution for <literal>internal</literal> and
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C-language functions, however.
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</para>
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<para>
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The full <acronym>SQL92</acronym> type syntax is allowed for
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input arguments and return value. However, some details of the
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type specification (e.g. the precision field for
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<type>numeric</type> types) are the responsibility of the
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underlying function implementation and are silently swallowed
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(e.g. not recognized or
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enforced) by the <command>CREATE FUNCTION</command> command.
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</para>
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<para>
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Two <literal>internal</literal>
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functions cannot have the same C name without causing
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errors at link time. To get around that, give them different C names
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(for example, use the argument types as part of the C names), then
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specify those names in the AS clause of <command>CREATE FUNCTION</command>.
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If the AS clause is left empty then <command>CREATE FUNCTION</command>
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assumes the C name of the function is the same as the SQL name.
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</para>
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<para>
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When overloading SQL functions with C-language functions, give
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each C-language instance of the function a distinct name, and use
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the alternative form of the <command>AS</command> clause in the
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<command>CREATE FUNCTION</command> syntax to ensure that
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overloaded SQL functions names are resolved to the correct
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dynamically linked objects.
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</para>
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<para>
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A C function cannot return a set of values.
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</para>
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</refsect2>
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</refsect1>
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<refsect1 id="R1-SQL-CREATEFUNCTION-2">
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<title>
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Usage
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</title>
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<para>
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To create a simple SQL function:
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<programlisting>
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CREATE FUNCTION one() RETURNS int4
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AS 'SELECT 1 AS RESULT'
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LANGUAGE 'sql';
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SELECT one() AS answer;
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<computeroutput>
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answer
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--------
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1
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</computeroutput>
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</programlisting>
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</para>
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<para>
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This example creates a C function by calling a routine from a user-created
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shared library. This particular routine calculates a check
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digit and returns TRUE if the check digit in the function parameters
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is correct. It is intended for use in a CHECK contraint.
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</para>
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<programlisting>
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CREATE FUNCTION ean_checkdigit(bpchar, bpchar) RETURNS bool
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AS '/usr1/proj/bray/sql/funcs.so' LANGUAGE 'c';
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CREATE TABLE product (
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id char(8) PRIMARY KEY,
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eanprefix char(8) CHECK (eanprefix ~ '[0-9]{2}-[0-9]{5}')
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REFERENCES brandname(ean_prefix),
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eancode char(6) CHECK (eancode ~ '[0-9]{6}'),
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CONSTRAINT ean CHECK (ean_checkdigit(eanprefix, eancode))
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);
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</programlisting>
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<para>
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This example creates a function that does type conversion between the
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user defined type complex, and the internal type point. The
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function is implemented by a dynamically loaded object that was
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compiled from C source. For <productname>Postgres</productname> to
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find a type conversion function automatically, the sql function has
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to have the same name as the return type, and overloading is
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unavoidable. The function name is overloaded by using the second
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form of the <command>AS</command> clause in the SQL definition:
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</para>
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<programlisting>
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CREATE FUNCTION point(complex) RETURNS point
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AS '/home/bernie/pgsql/lib/complex.so', 'complex_to_point'
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LANGUAGE 'c';
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</programlisting>
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<para>
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The C declaration of the function is:
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</para>
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<programlisting>
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Point * complex_to_point (Complex *z)
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{
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Point *p;
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p = (Point *) palloc(sizeof(Point));
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p->x = z->x;
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p->y = z->y;
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return p;
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}
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</programlisting>
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</refsect1>
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<refsect1 id="R1-SQL-CREATEFUNCTION-4">
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<title>
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Compatibility
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</title>
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<refsect2 id="R2-SQL-CREATEFUNCTION-4">
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<refsect2info>
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<date>2000-03-25</date>
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</refsect2info>
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<title>
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SQL92
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</title>
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<para>
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<command>CREATE FUNCTION</command> is
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a <productname>Postgres</productname> language extension.
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</para>
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</refsect2>
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<refsect2 id="R2-SQL-CREATEFUNCTION-5">
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<refsect2info>
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<date>2000-03-25</date>
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</refsect2info>
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<title>
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SQL/PSM
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</title>
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<para>
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<note>
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<para>
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PSM stands for Persistent Stored Modules. It is a procedural
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language and it was originally hoped that PSM would be ratified
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as an official standard by late 1996. As of mid-1998, this
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has not yet happened, but it is hoped that PSM will
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eventually become a standard.
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</para>
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</note>
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SQL/PSM <command>CREATE FUNCTION</command> has the following syntax:
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<synopsis>
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CREATE FUNCTION <replaceable class="parameter">name</replaceable>
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( [ [ IN | OUT | INOUT ] <replaceable class="parameter">type</replaceable> [, ...] ] )
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RETURNS <replaceable class="parameter">rtype</replaceable>
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LANGUAGE '<replaceable class="parameter">langname</replaceable>'
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ESPECIFIC <replaceable class="parameter">routine</replaceable>
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<replaceable class="parameter">SQL-statement</replaceable>
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</synopsis>
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</para>
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</refsect2>
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</refsect1>
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</refentry>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode:sgml
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sgml-omittag:nil
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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sgml-parent-document:nil
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sgml-default-dtd-file:"../reference.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:("/usr/lib/sgml/catalog")
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sgml-local-ecat-files:nil
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End:
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-->
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