<!-- doc/src/sgml/extend.sgml -->

 <chapter id="extend">
  <title>Extending <acronym>SQL</acronym></title>

  <indexterm zone="extend">
   <primary>extending SQL</primary>
  </indexterm>

  <para>
   In  the  sections  that follow, we will discuss how you
   can extend the <productname>PostgreSQL</productname>
   <acronym>SQL</acronym> query language by adding:

   <itemizedlist spacing="compact" mark="bullet">
    <listitem>
     <para>
      functions (starting in <xref linkend="xfunc">)
     </para>
    </listitem>
    <listitem>
     <para>
      aggregates (starting in <xref linkend="xaggr">)
     </para>
    </listitem>
    <listitem>
     <para>
      data types (starting in <xref linkend="xtypes">)
     </para>
    </listitem>
    <listitem>
     <para>
      operators (starting in <xref linkend="xoper">)
     </para>
    </listitem>
    <listitem>
     <para>
      operator classes for indexes (starting in <xref linkend="xindex">)
     </para>
    </listitem>
    <listitem>
     <para>
      packages of related objects (starting in <xref linkend="extend-extensions">)
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <sect1 id="extend-how">
   <title>How Extensibility Works</title>

   <para>
    <productname>PostgreSQL</productname> is extensible because its operation  is
    catalog-driven.   If  you  are familiar with standard
    relational database systems, you know that  they  store  information
    about  databases,  tables,  columns,  etc., in what are
    commonly known as system catalogs.  (Some systems  call
    this  the data dictionary.)  The catalogs appear to the
    user as tables like any other, but  the  <acronym>DBMS</acronym>  stores
    its  internal  bookkeeping in them.  One key difference
    between <productname>PostgreSQL</productname> and  standard  relational database systems  is
    that <productname>PostgreSQL</productname> stores much more information in its
    catalogs: not only information about tables and  columns,
    but also information about data types, functions, access
    methods, and so on.  These tables can be  modified  by
    the  user, and since <productname>PostgreSQL</productname> bases its operation
    on these tables, this means that <productname>PostgreSQL</productname> can  be
    extended   by   users.    By  comparison,  conventional
    database systems can only be extended by changing hardcoded
    procedures in the source code or by loading modules
    specially written by the <acronym>DBMS</acronym> vendor.
   </para>

   <para>
    The <productname>PostgreSQL</productname> server can moreover
    incorporate user-written code into itself through dynamic loading.
    That is, the user can specify an object code file (e.g., a shared
    library) that implements a new type or function, and
    <productname>PostgreSQL</productname> will load it as required.
    Code written in <acronym>SQL</acronym> is even more trivial to add
    to the server.  This ability to modify its operation <quote>on the
    fly</quote> makes <productname>PostgreSQL</productname> uniquely
    suited for rapid prototyping of new applications and storage
    structures.
   </para>
  </sect1>

  <sect1 id="extend-type-system">
   <title>The <productname>PostgreSQL</productname> Type System</title>

   <indexterm zone="extend-type-system">
    <primary>base type</primary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>data type</primary>
    <secondary>base</secondary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>composite type</primary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>data type</primary>
    <secondary>composite</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> data types are divided into base
    types, composite types, domains, and pseudo-types.
   </para>

   <sect2>
    <title>Base Types</title>

    <para>
     Base types are those, like <type>int4</type>, that are
     implemented below the level of the <acronym>SQL</> language
     (typically in a low-level language such as C).  They generally
     correspond to what are often known as abstract data types.
     <productname>PostgreSQL</productname> can only operate on such
     types through functions provided by the user and only understands
     the behavior of such types to the extent that the user describes
     them.  Base types are further subdivided into scalar and array
     types.  For each scalar type, a corresponding array type is
     automatically created that can hold variable-size arrays of that
     scalar type.
    </para>
   </sect2>

   <sect2>
    <title>Composite Types</title>

    <para>
     Composite types, or row types, are created whenever the user
     creates a table. It is also possible to use <xref
     linkend="sql-createtype"> to
     define a <quote>stand-alone</> composite type with no associated
     table.  A composite type is simply a list of types with
     associated field names.  A value of a composite type is a row or
     record of field values.  The user can access the component fields
     from <acronym>SQL</> queries. Refer to <xref linkend="rowtypes">
     for more information on composite types.
    </para>
   </sect2>

   <sect2>
    <title>Domains</title>

    <para>
     A domain is based on a particular base type and for many purposes
     is interchangeable with its base type.  However, a domain can
     have constraints that restrict its valid values to a subset of
     what the underlying base type would allow.
    </para>

    <para>
     Domains can be created using the <acronym>SQL</> command
     <xref linkend="sql-createdomain">.
     Their creation and use is not discussed in this chapter.
    </para>
   </sect2>

   <sect2>
    <title>Pseudo-Types</title>

    <para>
     There are a few <quote>pseudo-types</> for special purposes.
     Pseudo-types cannot appear as columns of tables or attributes of
     composite types, but they can be used to declare the argument and
     result types of functions.  This provides a mechanism within the
     type system to identify special classes of functions.  <xref
     linkend="datatype-pseudotypes-table"> lists the existing
     pseudo-types.
    </para>
   </sect2>

   <sect2 id="extend-types-polymorphic">
    <title>Polymorphic Types</title>

   <indexterm zone="extend-types-polymorphic">
    <primary>polymorphic type</primary>
   </indexterm>

   <indexterm zone="extend-types-polymorphic">
    <primary>polymorphic function</primary>
   </indexterm>

   <indexterm zone="extend-types-polymorphic">
    <primary>type</primary>
    <secondary>polymorphic</secondary>
   </indexterm>

   <indexterm zone="extend-types-polymorphic">
    <primary>function</primary>
    <secondary>polymorphic</secondary>
   </indexterm>

    <para>
     Four pseudo-types of special interest are <type>anyelement</>,
     <type>anyarray</>, <type>anynonarray</>, and <type>anyenum</>,
     which are collectively called <firstterm>polymorphic types</>.
     Any function declared using these types is said to be
     a <firstterm>polymorphic function</>.  A polymorphic function can
     operate on many different data types, with the specific data type(s)
     being determined by the data types actually passed to it in a particular
     call.
    </para>

    <para>
     Polymorphic arguments and results are tied to each other and are resolved
     to a specific data type when a query calling a polymorphic function is
     parsed.  Each position (either argument or return value) declared as
     <type>anyelement</type> is allowed to have any specific actual
     data type, but in any given call they must all be the
     <emphasis>same</emphasis> actual type. Each
     position declared as <type>anyarray</type> can have any array data type,
     but similarly they must all be the same type. If there are
     positions declared <type>anyarray</type> and others declared
     <type>anyelement</type>, the actual array type in the
     <type>anyarray</type> positions must be an array whose elements are
     the same type appearing in the <type>anyelement</type> positions.
     <type>anynonarray</> is treated exactly the same as <type>anyelement</>,
     but adds the additional constraint that the actual type must not be
     an array type.
     <type>anyenum</> is treated exactly the same as <type>anyelement</>,
     but adds the additional constraint that the actual type must
     be an enum type.
    </para>

    <para>
     Thus, when more than one argument position is declared with a polymorphic
     type, the net effect is that only certain combinations of actual argument
     types are allowed.  For example, a function declared as
     <literal>equal(anyelement, anyelement)</> will take any two input values,
     so long as they are of the same data type.
    </para>

    <para>
     When the return value of a function is declared as a polymorphic type,
     there must be at least one argument position that is also polymorphic,
     and the actual data type supplied as the argument determines the actual
     result type for that call.  For example, if there were not already
     an array subscripting mechanism, one could define a function that
     implements subscripting as <literal>subscript(anyarray, integer)
     returns anyelement</>.  This declaration constrains the actual first
     argument to be an array type, and allows the parser to infer the correct
     result type from the actual first argument's type.  Another example
     is that a function declared as <literal>f(anyarray) returns anyenum</>
     will only accept arrays of enum types.
    </para>

    <para>
     Note that <type>anynonarray</> and <type>anyenum</> do not represent
     separate type variables; they are the same type as
     <type>anyelement</type>, just with an additional constraint.  For
     example, declaring a function as <literal>f(anyelement, anyenum)</>
     is equivalent to declaring it as <literal>f(anyenum, anyenum)</>:
     both actual arguments have to be the same enum type.
    </para>

    <para>
     A variadic function (one taking a variable number of arguments, as in
     <xref linkend="xfunc-sql-variadic-functions">) can be
     polymorphic: this is accomplished by declaring its last parameter as
     <literal>VARIADIC</> <type>anyarray</>.  For purposes of argument
     matching and determining the actual result type, such a function behaves
     the same as if you had written the appropriate number of
     <type>anynonarray</> parameters.
    </para>
   </sect2>
  </sect1>

  &xfunc;
  &xaggr;
  &xtypes;
  &xoper;
  &xindex;


  <sect1 id="extend-extensions">
   <title>Packaging Related Objects into an Extension</title>

   <indexterm zone="extend-extensions">
    <primary>extension</primary>
   </indexterm>

   <para>
    A useful extension to <productname>PostgreSQL</> typically includes
    multiple SQL objects; for example, a new datatype will require new
    functions, new operators, and probably new index operator classes.
    It is helpful to collect all these objects into a single package
    to simplify database management.  <productname>PostgreSQL</> calls
    such a package an <firstterm>extension</>.  To define an extension,
    you need at least a <firstterm>script file</> that contains the
    <acronym>SQL</> commands to create the extension's objects, and a
    <firstterm>control file</> that specifies a few basic properties
    of the extension itself.  If the extension includes C code, there
    will typically also be a shared library file into which the C code
    has been built.  Once you have these files, a simple
    <xref linkend="sql-createextension"> command loads the objects into
    your database.
   </para>

   <para>
    The main advantage of using an extension, rather than just running the
    <acronym>SQL</> script to load a bunch of <quote>loose</> objects
    into your database, is that <productname>PostgreSQL</> will then
    understand that the objects of the extension go together.  You can
    drop all the objects with a single <xref linkend="sql-dropextension">
    command (no need to maintain a separate <quote>uninstall</> script).
    Even more useful, <application>pg_dump</> knows that it should not
    dump the individual member objects of the extension &mdash; it will
    just include a <command>CREATE EXTENSION</> command in dumps, instead.
    This vastly simplifies migration to a new version of the extension
    that might contain more or different objects than the old version.
    Note however that you must have the extension's control, script, and
    other files available when loading such a dump into a new database.
   </para>

   <para>
    <productname>PostgreSQL</> will not let you drop an individual object
    contained in an extension, except by dropping the whole extension.
    Also, while you can change the definition of an extension member object
    (for example, via <command>CREATE OR REPLACE FUNCTION</command> for a
    function), bear in mind that the modified definition will not be dumped
    by <application>pg_dump</>.  Such a change is usually only sensible if
    you concurrently make the same change in the extension's script file.
    (But there are special provisions for tables containing configuration
    data; see below.)
   </para>

   <para>
    The extension mechanism also has provisions for packaging modification
    scripts that adjust the definitions of the SQL objects contained in an
    extension.  For example, if version 1.1 of an extension adds one function
    and changes the body of another function compared to 1.0, the extension
    author can provide an <firstterm>update script</> that makes just those
    two changes.  The <command>ALTER EXTENSION UPDATE</> command can then
    be used to apply these changes and track which version of the extension
    is actually installed in a given database.
   </para>

   <para>
    The kinds of SQL objects that can be members of an extension are shown in
    the description of <xref linkend="sql-alterextension">.  Notably, objects
    that are database-cluster-wide, such as databases, roles, and tablespaces,
    cannot be extension members since an extension is only known within one
    database.  (Although an extension script is not prohibited from creating
    such objects, if it does so they will not be tracked as part of the
    extension.)  Also notice that while a table can be a member of an
    extension, its subsidiary objects such as indexes are not directly
    considered members of the extension.
   </para>

   <sect2 id="extension">
    <title>Extension Files</title>

   <indexterm>
    <primary>control file</primary>
   </indexterm>

    <para>
     The <xref linkend="sql-createextension"> command relies on a control
     file for each extension, which must be named the same as the extension
     with a suffix of <literal>.control</>, and must be placed in the
     installation's <literal>SHAREDIR/extension</literal> directory.  There
     must also be at least one <acronym>SQL</> script file, which follows the
     naming pattern
     <literal><replaceable>extension</>--<replaceable>version</>.sql</literal>
     (for example, <literal>foo--1.0.sql</> for version <literal>1.0</> of
     extension <literal>foo</>).  By default, the script file(s) are also
     placed in the <literal>SHAREDIR/extension</literal> directory; but the
     control file can specify a different directory for the script file(s).
    </para>

    <para>
     The file format for an extension control file is the same as for the
     <filename>postgresql.conf</> file, namely a list of
     <replaceable>parameter_name</> <literal>=</> <replaceable>value</>
     assignments, one per line.  Blank lines and comments introduced by
     <literal>#</> are allowed.  Be sure to quote any value that is not
     a single word or number.
    </para>

    <para>
     A control file can set the following parameters:
    </para>

    <variablelist>
     <varlistentry>
      <term><varname>directory</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        The directory containing the extension's <acronym>SQL</> script
        file(s).  Unless an absolute path is given, the name is relative to
        the installation's <literal>SHAREDIR</literal> directory.  The
        default behavior is equivalent to specifying
        <literal>directory = 'extension'</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>default_version</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        The default version of the extension (the one that will be installed
        if no version is specified in <command>CREATE EXTENSION</>).  Although
        this can be omitted, that will result in <command>CREATE EXTENSION</>
        failing if no <literal>VERSION</> option appears, so you generally
        don't want to do that.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>comment</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        A comment (any string) about the extension.  Alternatively,
        the comment can be set by means of the <xref linkend="sql-comment">
        command in the script file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>encoding</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        The character set encoding used by the script file(s).  This should
        be specified if the script files contain any non-ASCII characters.
        Otherwise the files will be assumed to be in the database encoding.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>module_pathname</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        The value of this parameter will be substituted for each occurrence
        of <literal>MODULE_PATHNAME</> in the script file(s).  If it is not
        set, no substitution is made.  Typically, this is set to
        <literal>$libdir/<replaceable>shared_library_name</></literal> and
        then <literal>MODULE_PATHNAME</> is used in <command>CREATE
        FUNCTION</> commands for C-language functions, so that the script
        files do not need to hard-wire the name of the shared library.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>requires</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        A list of names of extensions that this extension depends on,
        for example <literal>requires = 'foo, bar'</literal>.  Those
        extensions must be installed before this one can be installed.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>superuser</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        If this parameter is <literal>true</> (which is the default),
        only superusers can create the extension or update it to a new
        version.  If it is set to <literal>false</>, just the privileges
        required to execute the commands in the installation or update script
        are required.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>relocatable</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        An extension is <firstterm>relocatable</> if it is possible to move
        its contained objects into a different schema after initial creation
        of the extension.  The default is <literal>false</>, i.e. the
        extension is not relocatable.
        See below for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>schema</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        This parameter can only be set for non-relocatable extensions.
        It forces the extension to be loaded into exactly the named schema
        and not any other.  See below for more information.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

    <para>
     In addition to the primary control file
     <literal><replaceable>extension</>.control</literal>,
     an extension can have secondary control files named in the style
     <literal><replaceable>extension</>--<replaceable>version</>.control</literal>.
     If supplied, these must be located in the script file directory.
     Secondary control files follow the same format as the primary control
     file.  Any parameters set in a secondary control file override the
     primary control file when installing or updating to that version of
     the extension.  However, the parameters <varname>directory</> and
     <varname>default_version</> cannot be set in a secondary control file.
    </para>

    <para>
     An extension's <acronym>SQL</> script files can contain any SQL commands,
     except for transaction control commands (<command>BEGIN</>,
     <command>COMMIT</>, etc) and commands that cannot be executed inside a
     transaction block (such as <command>VACUUM</>).  This is because the
     script files are implicitly executed within a transaction block.
    </para>

    <para>
     While the script files can contain any characters allowed by the specified
     encoding, control files should contain only plain ASCII, because there
     is no way for <productname>PostgreSQL</> to know what encoding a
     control file is in.  In practice this is only an issue if you want to
     use non-ASCII characters in the extension's comment.  Recommended
     practice in that case is to not use the control file <varname>comment</>
     parameter, but instead use <command>COMMENT ON EXTENSION</>
     within a script file to set the comment.
    </para>

   </sect2>

   <sect2>
    <title>Extension Relocatability</title>

    <para>
     Users often wish to load the objects contained in an extension into a
     different schema than the extension's author had in mind.  There are
     three supported levels of relocatability:
    </para>

    <itemizedlist>
     <listitem>
      <para>
       A fully relocatable extension can be moved into another schema
       at any time, even after it's been loaded into a database.
       This is done with the <command>ALTER EXTENSION SET SCHEMA</>
       command, which automatically renames all the member objects into
       the new schema.  Normally, this is only possible if the extension
       contains no internal assumptions about what schema any of its
       objects are in.  Also, the extension's objects must all be in one
       schema to begin with (ignoring objects that do not belong to any
       schema, such as procedural languages).  Mark a fully relocatable
       extension by setting <literal>relocatable = true</> in its control
       file.
      </para>
     </listitem>

     <listitem>
      <para>
       An extension might be relocatable during installation but not
       afterwards.  This is typically the case if the extension's script
       file needs to reference the target schema explicitly, for example
       in setting <literal>search_path</> properties for SQL functions.
       For such an extension, set <literal>relocatable = false</> in its
       control file, and use <literal>@extschema@</> to refer to the target
       schema in the script file.  All occurrences of this string will be
       replaced by the actual target schema's name before the script is
       executed.  The user can set the target schema using the
       <literal>SCHEMA</> option of <command>CREATE EXTENSION</>.
      </para>
     </listitem>

     <listitem>
      <para>
       If the extension does not support relocation at all, set
       <literal>relocatable = false</> in its control file, and also set
       <literal>schema</> to the name of the intended target schema.  This
       will prevent use of the <literal>SCHEMA</> option of <command>CREATE
       EXTENSION</>, unless it specifies the same schema named in the control
       file.  This choice is typically necessary if the extension contains
       internal assumptions about schema names that can't be replaced by
       uses of <literal>@extschema@</>.  The <literal>@extschema@</>
       substitution mechanism is available in this case too, although it is
       of limited use since the schema name is determined by the control file.
      </para>
     </listitem>
    </itemizedlist>

    <para>
     In all cases, the script file will be executed with
     <xref linkend="guc-search-path"> initially set to point to the target
     schema; that is, <command>CREATE EXTENSION</> does the equivalent of
     this:
<programlisting>
SET LOCAL search_path TO @extschema@;
</programlisting>
     This allows the objects created by the script file to go into the target
     schema.  The script file can change <varname>search_path</> if it wishes,
     but that is generally undesirable.  <varname>search_path</> is restored
     to its previous setting upon completion of <command>CREATE EXTENSION</>.
    </para>

    <para>
     The target schema is determined by the <varname>schema</> parameter in
     the control file if that is given, otherwise by the <literal>SCHEMA</>
     option of <command>CREATE EXTENSION</> if that is given, otherwise the
     current default object creation schema (the first one in the caller's
     <varname>search_path</>).  When the control file <varname>schema</>
     parameter is used, the target schema will be created if it doesn't
     already exist, but in the other two cases it must already exist.
    </para>

    <para>
     If any prerequisite extensions are listed in <varname>requires</varname>
     in the control file, their target schemas are appended to the initial
     setting of <varname>search_path</>.  This allows their objects to be
     visible to the new extension's script file.
    </para>

    <para>
     Although a non-relocatable extension can contain objects spread across
     multiple schemas, it is usually desirable to place all the objects meant
     for external use into a single schema, which is considered the extension's
     target schema.  Such an arrangement works conveniently with the default
     setting of <varname>search_path</> during creation of dependent
     extensions.
    </para>
   </sect2>

   <sect2>
    <title>Extension Configuration Tables</title>

    <para>
     Some extensions include configuration tables, which contain data that
     might be added or changed by the user after installation of the
     extension.  Ordinarily, if a table is part of an extension, neither
     the table's definition nor its content will be dumped by
     <application>pg_dump</>.  But that behavior is undesirable for a
     configuration table; any data changes made by the user need to be
     included in dumps, or the extension will behave differently after a dump
     and reload.
    </para>

    <para>
     To solve this problem, an extension's script file can mark a table
     it has created as a configuration table, which will cause
     <application>pg_dump</> to include the table's contents (not its
     definition) in dumps.  To do that, call the function
     <function>pg_extension_config_dump(regclass, text)</> after creating the
     table, for example
<programlisting>
CREATE TABLE my_config (key text, value text);

SELECT pg_catalog.pg_extension_config_dump('my_config', '');
</programlisting>
     Any number of tables can be marked this way.
    </para>

    <para>
     When the second argument of <function>pg_extension_config_dump</> is
     an empty string, the entire contents of the table are dumped by
     <application>pg_dump</>.  This is usually only correct if the table
     is initially empty as created by the extension script.  If there is
     a mixture of initial data and user-provided data in the table,
     the second argument of <function>pg_extension_config_dump</> provides
     a <literal>WHERE</> condition that selects the data to be dumped.
     For example, you might do
<programlisting>
CREATE TABLE my_config (key text, value text, standard_entry boolean);

SELECT pg_catalog.pg_extension_config_dump('my_config', 'WHERE NOT standard_entry');
</programlisting>
     and then make sure that <structfield>standard_entry</> is true only
     in the rows created by the extension's script.
    </para>

    <para>
     More complicated situations, such as initially-provided rows that might
     be modified by users, can be handled by creating triggers on the
     configuration table to ensure that modified rows are marked correctly.
    </para>
   </sect2>

   <sect2>
    <title>Extension Updates</title>

    <para>
     One advantage of the extension mechanism is that it provides convenient
     ways to manage updates to the SQL commands that define an extension's
     objects.  This is done by associating a version name or number with
     each released version of the extension's installation script.
     In addition, if you want users to be able to update their databases
     dynamically from one version to the next, you should provide
     <firstterm>update scripts</> that make the necessary changes to go from
     one version to the next.  Update scripts have names following the pattern
     <literal><replaceable>extension</>--<replaceable>oldversion</>--<replaceable>newversion</>.sql</literal>
     (for example, <literal>foo--1.0--1.1.sql</> contains the commands to modify
     version <literal>1.0</> of extension <literal>foo</> into version
     <literal>1.1</>).
    </para>

    <para>
     Given that a suitable update script is available, the command
     <command>ALTER EXTENSION UPDATE</> will update an installed extension
     to the specified new version.  The update script is run in the same
     environment that <command>CREATE EXTENSION</> provides for installation
     scripts: in particular, <varname>search_path</> is set up in the same
     way, and any new objects created by the script are automatically added
     to the extension.
    </para>

    <para>
     If an extension has secondary control files, the control parameters
     that are used for an update script are those associated with the script's
     target (new) version.
    </para>

    <para>
     The update mechanism can be used to solve an important special case:
     converting a <quote>loose</> collection of objects into an extension.
     Before the extension mechanism was added to
     <productname>PostgreSQL</productname> (in 9.1), many people wrote
     extension modules that simply created assorted unpackaged objects.
     Given an existing database containing such objects, how can we convert
     the objects into a properly packaged extension?  Dropping them and then
     doing a plain <command>CREATE EXTENSION</> is one way, but it's not
     desirable if the objects have dependencies (for example, if there are
     table columns of a data type created by the extension).  The way to fix
     this situation is to create an empty extension, then use <command>ALTER
     EXTENSION ADD</> to attach each pre-existing object to the extension,
     then finally create any new objects that are in the current extension
     version but were not in the unpackaged release.  <command>CREATE
     EXTENSION</> supports this case with its <literal>FROM</> <replaceable
     class="parameter">old_version</> option, which causes it to not run the
     normal installation script for the target version, but instead the update
     script named
     <literal><replaceable>extension</>--<replaceable>old_version</>--<replaceable>target_version</>.sql</literal>.
     The choice of the dummy version name to use as <replaceable
     class="parameter">old_version</> is up to the extension author, though
     <literal>unpackaged</> is a common convention.  If you have multiple
     prior versions you need to be able to update into extension style, use
     multiple dummy version names to identify them.
    </para>

    <para>
     <command>ALTER EXTENSION</> is able to execute sequences of update
     script files to achieve a requested update.  For example, if only
     <literal>foo--1.0--1.1.sql</> and <literal>foo--1.1--2.0.sql</> are
     available, <command>ALTER EXTENSION</> will apply them in sequence if an
     update to version <literal>2.0</> is requested when <literal>1.0</> is
     currently installed.
    </para>

    <para>
     <productname>PostgreSQL</> doesn't assume anything about the properties
     of version names: for example, it does not know whether <literal>1.1</>
     follows <literal>1.0</>.  It just matches up the available version names
     and follows the path that requires applying the fewest update scripts.
     (A version name can actually be any string that doesn't contain
     <literal>--</> or leading or trailing <literal>-</>.)
    </para>

    <para>
     Sometimes it is useful to provide <quote>downgrade</> scripts, for
     example <literal>foo--1.1--1.0.sql</> to allow reverting the changes
     associated with version <literal>1.1</>.  If you do that, be careful
     of the possibility that a downgrade script might unexpectedly
     get applied because it yields a shorter path.  The risky case is where
     there is a <quote>fast path</> update script that jumps ahead several
     versions as well as a downgrade script to the fast path's start point.
     It might take fewer steps to apply the downgrade and then the fast
     path than to move ahead one version at a time.  If the downgrade script
     drops any irreplaceable objects, this will yield undesirable results.
    </para>

    <para>
     To check for unexpected update paths, use this command:
<programlisting>
SELECT * FROM pg_extension_update_paths('<replaceable>extension_name</>');
</programlisting>
     This shows each pair of distinct known version names for the specified
     extension, together with the update path sequence that would be taken to
     get from the source version to the target version, or <literal>NULL</> if
     there is no available update path.  The path is shown in textual form
     with <literal>--</> separators.  You can use
     <literal>regexp_split_to_array(path,'--')</> if you prefer an array
     format.
    </para>
   </sect2>

   <sect2>
    <title>Extension Example</title>

    <para>
     Here is a complete example of an <acronym>SQL</>-only
     extension, a two-element composite type that can store any type of value
     in its slots, which are named <quote>k</> and <quote>v</>.  Non-text
     values are automatically coerced to text for storage.
    </para>

    <para>
     The script file <filename>pair--1.0.sql</> looks like this:

<programlisting><![CDATA[
CREATE TYPE pair AS ( k text, v text );

CREATE OR REPLACE FUNCTION pair(anyelement, text)
RETURNS pair LANGUAGE SQL AS 'SELECT ROW($1, $2)::pair';

CREATE OR REPLACE FUNCTION pair(text, anyelement)
RETURNS pair LANGUAGE SQL AS 'SELECT ROW($1, $2)::pair';

CREATE OR REPLACE FUNCTION pair(anyelement, anyelement)
RETURNS pair LANGUAGE SQL AS 'SELECT ROW($1, $2)::pair';

CREATE OR REPLACE FUNCTION pair(text, text)
RETURNS pair LANGUAGE SQL AS 'SELECT ROW($1, $2)::pair;';

CREATE OPERATOR ~> (LEFTARG = text, RIGHTARG = anyelement, PROCEDURE = pair);
CREATE OPERATOR ~> (LEFTARG = anyelement, RIGHTARG = text, PROCEDURE = pair);
CREATE OPERATOR ~> (LEFTARG = anyelement, RIGHTARG = anyelement, PROCEDURE = pair);
CREATE OPERATOR ~> (LEFTARG = text, RIGHTARG = text, PROCEDURE = pair);
]]>
</programlisting>
    </para>

    <para>
     The control file <filename>pair.control</> looks like this:

<programlisting>
# pair extension
comment = 'A key/value pair data type'
default_version = '1.0'
relocatable = true
</programlisting>
    </para>

    <para>
     While you hardly need a makefile to install these two files into the
     correct directory, you could use a <filename>Makefile</> containing this:

<programlisting>
EXTENSION = pair
DATA = pair--1.0.sql

PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
</programlisting>

     This makefile relies on <acronym>PGXS</acronym>, which is described
     in <xref linkend="extend-pgxs">.  The command <literal>make install</>
     will install the control and script files into the correct
     directory as reported by <application>pg_config</>.
    </para>

    <para>
     Once the files are installed, use the
     <xref linkend="sql-createextension"> command to load the objects into
     any particular database.
    </para>
   </sect2>
  </sect1>

  <sect1 id="extend-pgxs">
   <title>Extension Building Infrastructure</title>

   <indexterm zone="extend-pgxs">
    <primary>pgxs</primary>
   </indexterm>

   <para>
    If you are thinking about distributing your
    <productname>PostgreSQL</> extension modules, setting up a
    portable build system for them can be fairly difficult.  Therefore
    the <productname>PostgreSQL</> installation provides a build
    infrastructure for extensions, called <acronym>PGXS</acronym>, so
    that simple extension modules can be built simply against an
    already installed server.  <acronym>PGXS</acronym> is mainly intended
    for extensions that include C code, although it can be used for
    pure-SQL extensions too.  Note that <acronym>PGXS</acronym> is not
    intended to be a universal build system framework that can be used
    to build any software interfacing to <productname>PostgreSQL</>;
    it simply automates common build rules for simple server extension
    modules.  For more complicated packages, you might need to write your
    own build system.
   </para>

   <para>
    To use the <acronym>PGXS</acronym> infrastructure for your extension,
    you must write a simple makefile.
    In the makefile, you need to set some variables
    and finally include the global <acronym>PGXS</acronym> makefile.
    Here is an example that builds an extension module named
    <literal>isbn_issn</literal>, consisting of a shared library containing
    some C code, an extension control file, a SQL script, and a documentation
    text file:
<programlisting>
MODULES = isbn_issn
EXTENSION = isbn_issn
DATA = isbn_issn--1.0.sql
DOCS = README.isbn_issn

PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
</programlisting>
    The last three lines should always be the same.  Earlier in the
    file, you assign variables or add custom
    <application>make</application> rules.
   </para>

   <para>
    Set one of these three variables to specify what is built:

    <variablelist>
     <varlistentry>
      <term><varname>MODULES</varname></term>
      <listitem>
       <para>
        list of shared-library objects to be built from source files with same
        stem (do not include library suffixes in this list)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>MODULE_big</varname></term>
      <listitem>
       <para>
        a shared library to build from multiple source files
        (list object files in <varname>OBJS</varname>)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>PROGRAM</varname></term>
      <listitem>
       <para>
        an executable program to build
        (list object files in <varname>OBJS</varname>)
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

    The following variables can also be set:

    <variablelist>
     <varlistentry>
      <term><varname>EXTENSION</varname></term>
      <listitem>
       <para>
        extension name(s); for each name you must provide an
        <literal><replaceable>extension</replaceable>.control</literal> file,
        which will be installed into
        <literal><replaceable>prefix</replaceable>/share/extension</literal>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>MODULEDIR</varname></term>
      <listitem>
       <para>
        subdirectory of <literal><replaceable>prefix</>/share</literal>
        into which DATA and DOCS files should be installed
        (if not set, default is <literal>extension</literal> if
        <varname>EXTENSION</varname> is set,
        or <literal>contrib</literal> if not)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>DATA</varname></term>
      <listitem>
       <para>
        random files to install into <literal><replaceable>prefix</replaceable>/share/$MODULEDIR</literal>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>DATA_built</varname></term>
      <listitem>
       <para>
        random files to install into
        <literal><replaceable>prefix</replaceable>/share/$MODULEDIR</literal>,
        which need to be built first
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>DATA_TSEARCH</varname></term>
      <listitem>
       <para>
        random files to install under
        <literal><replaceable>prefix</replaceable>/share/tsearch_data</literal>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>DOCS</varname></term>
      <listitem>
       <para>
        random files to install under
        <literal><replaceable>prefix</replaceable>/doc/$MODULEDIR</literal>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>SCRIPTS</varname></term>
      <listitem>
       <para>
        script files (not binaries) to install into
        <literal><replaceable>prefix</replaceable>/bin</literal>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>SCRIPTS_built</varname></term>
      <listitem>
       <para>
        script files (not binaries) to install into
        <literal><replaceable>prefix</replaceable>/bin</literal>,
        which need to be built first
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>REGRESS</varname></term>
      <listitem>
       <para>
        list of regression test cases (without suffix), see below
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>EXTRA_CLEAN</varname></term>
      <listitem>
       <para>
        extra files to remove in <literal>make clean</literal>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>PG_CPPFLAGS</varname></term>
      <listitem>
       <para>
        will be added to <varname>CPPFLAGS</varname>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>PG_LIBS</varname></term>
      <listitem>
       <para>
        will be added to <varname>PROGRAM</varname> link line
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>SHLIB_LINK</varname></term>
      <listitem>
       <para>
        will be added to <varname>MODULE_big</varname> link line
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>PG_CONFIG</varname></term>
      <listitem>
       <para>
        path to <application>pg_config</> program for the
        <productname>PostgreSQL</productname> installation to build against
        (typically just <literal>pg_config</> to use the first one in your
        <varname>PATH</>)
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    Put this makefile as <literal>Makefile</literal> in the directory
    which holds your extension. Then you can do
    <literal>make</literal> to compile, and then <literal>make
    install</literal> to install your module.  By default, the extension is
    compiled and installed for the
    <productname>PostgreSQL</productname> installation that
    corresponds to the first <command>pg_config</command> program
    found in your <varname>PATH</>.  You can use a different installation by
    setting <varname>PG_CONFIG</varname> to point to its
    <command>pg_config</command> program, either within the makefile
    or on the <literal>make</literal> command line.
   </para>

   <caution>
    <para>
     Changing <varname>PG_CONFIG</varname> only works when building
     against <productname>PostgreSQL</productname> 8.3 or later.
     With older releases it does not work to set it to anything except
     <literal>pg_config</>; you must alter your <varname>PATH</>
     to select the installation to build against.
    </para>
   </caution>

   <para>
    The scripts listed in the <varname>REGRESS</> variable are used for
    regression testing of your module, which can be invoked by <literal>make
    installcheck</literal> after doing <literal>make install</>.  For this to
    work you must have a running <productname>PostgreSQL</productname> server.
    The script files listed in <varname>REGRESS</> must appear in a
    subdirectory named <literal>sql/</literal> in your extension's directory.
    These files must have extension <literal>.sql</literal>, which must not be
    included in the <varname>REGRESS</varname> list in the makefile.  For each
    test there should also be a file containing the expected output in a
    subdirectory named <literal>expected/</literal>, with the same stem and
    extension <literal>.out</literal>.  <literal>make installcheck</literal>
    executes each test script with <application>psql</>, and compares the
    resulting output to the matching expected file.  Any differences will be
    written to the file <literal>regression.diffs</literal> in <command>diff
    -c</command> format.  Note that trying to run a test that is missing its
    expected file will be reported as <quote>trouble</quote>, so make sure you
    have all expected files.
   </para>

   <tip>
    <para>
     The easiest way to create the expected files is to create empty files,
     then do a test run (which will of course report differences).  Inspect
     the actual result files found in the <literal>results/</literal>
     directory, then copy them to <literal>expected/</literal> if they match
     what you expect from the test.
    </para>

   </tip>
  </sect1>

 </chapter>
