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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="EXTEND-EXTENSIONS"
>35.15. Packaging Related Objects into an Extension</A
></H1
><P
> A useful extension to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> typically includes
multiple SQL objects; for example, a new data type 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. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> calls
such a package an <I
CLASS="FIRSTTERM"
>extension</I
>. To define an extension,
you need at least a <I
CLASS="FIRSTTERM"
>script file</I
> that contains the
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> commands to create the extension's objects, and a
<I
CLASS="FIRSTTERM"
>control file</I
> 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
<A
HREF="sql-createextension.html"
>CREATE EXTENSION</A
> command loads the objects into
your database.
</P
><P
> The main advantage of using an extension, rather than just running the
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> script to load a bunch of <SPAN
CLASS="QUOTE"
>"loose"</SPAN
> objects
into your database, is that <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will then
understand that the objects of the extension go together. You can
drop all the objects with a single <A
HREF="sql-dropextension.html"
>DROP EXTENSION</A
>
command (no need to maintain a separate <SPAN
CLASS="QUOTE"
>"uninstall"</SPAN
> script).
Even more useful, <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> knows that it should not
dump the individual member objects of the extension — it will
just include a <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> 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.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 <TT
CLASS="COMMAND"
>CREATE OR REPLACE FUNCTION</TT
> for a
function), bear in mind that the modified definition will not be dumped
by <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>. 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.)
</P
><P
> 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 <I
CLASS="FIRSTTERM"
>update script</I
> that makes just those
two changes. The <TT
CLASS="COMMAND"
>ALTER EXTENSION UPDATE</TT
> command can then
be used to apply these changes and track which version of the extension
is actually installed in a given database.
</P
><P
> The kinds of SQL objects that can be members of an extension are shown in
the description of <A
HREF="sql-alterextension.html"
>ALTER EXTENSION</A
>. 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.
Another important point is that schemas can belong to extensions, but not
vice versa: an extension as such has an unqualified name and does not
exist <SPAN
CLASS="QUOTE"
>"within"</SPAN
> any schema. The extension's member objects,
however, will belong to schemas whenever appropriate for their object
types. It may or may not be appropriate for an extension to own the
schema(s) its member objects are within.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN54518"
>35.15.1. Extension Files</A
></H2
><P
> The <A
HREF="sql-createextension.html"
>CREATE EXTENSION</A
> command relies on a control
file for each extension, which must be named the same as the extension
with a suffix of <TT
CLASS="LITERAL"
>.control</TT
>, and must be placed in the
installation's <TT
CLASS="LITERAL"
>SHAREDIR/extension</TT
> directory. There
must also be at least one <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> script file, which follows the
naming pattern
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>extension</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>version</I
></TT
>.sql</TT
>
(for example, <TT
CLASS="LITERAL"
>foo--1.0.sql</TT
> for version <TT
CLASS="LITERAL"
>1.0</TT
> of
extension <TT
CLASS="LITERAL"
>foo</TT
>). By default, the script file(s) are also
placed in the <TT
CLASS="LITERAL"
>SHAREDIR/extension</TT
> directory; but the
control file can specify a different directory for the script file(s).
</P
><P
> The file format for an extension control file is the same as for the
<TT
CLASS="FILENAME"
>postgresql.conf</TT
> file, namely a list of
<TT
CLASS="REPLACEABLE"
><I
>parameter_name</I
></TT
> <TT
CLASS="LITERAL"
>=</TT
> <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
>
assignments, one per line. Blank lines and comments introduced by
<TT
CLASS="LITERAL"
>#</TT
> are allowed. Be sure to quote any value that is not
a single word or number.
</P
><P
> A control file can set the following parameters:
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="VARNAME"
>directory</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
> The directory containing the extension's <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> script
file(s). Unless an absolute path is given, the name is relative to
the installation's <TT
CLASS="LITERAL"
>SHAREDIR</TT
> directory. The
default behavior is equivalent to specifying
<TT
CLASS="LITERAL"
>directory = 'extension'</TT
>.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>default_version</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
> The default version of the extension (the one that will be installed
if no version is specified in <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
>). Although
this can be omitted, that will result in <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
>
failing if no <TT
CLASS="LITERAL"
>VERSION</TT
> option appears, so you generally
don't want to do that.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>comment</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
> A comment (any string) about the extension. Alternatively,
the comment can be set by means of the <A
HREF="sql-comment.html"
>COMMENT</A
>
command in the script file.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>encoding</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
> 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.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>module_pathname</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
> The value of this parameter will be substituted for each occurrence
of <TT
CLASS="LITERAL"
>MODULE_PATHNAME</TT
> in the script file(s). If it is not
set, no substitution is made. Typically, this is set to
<TT
CLASS="LITERAL"
>$libdir/<TT
CLASS="REPLACEABLE"
><I
>shared_library_name</I
></TT
></TT
> and
then <TT
CLASS="LITERAL"
>MODULE_PATHNAME</TT
> is used in <TT
CLASS="COMMAND"
>CREATE
FUNCTION</TT
> commands for C-language functions, so that the script
files do not need to hard-wire the name of the shared library.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>requires</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
> A list of names of extensions that this extension depends on,
for example <TT
CLASS="LITERAL"
>requires = 'foo, bar'</TT
>. Those
extensions must be installed before this one can be installed.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>superuser</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> If this parameter is <TT
CLASS="LITERAL"
>true</TT
> (which is the default),
only superusers can create the extension or update it to a new
version. If it is set to <TT
CLASS="LITERAL"
>false</TT
>, just the privileges
required to execute the commands in the installation or update script
are required.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>relocatable</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> An extension is <I
CLASS="FIRSTTERM"
>relocatable</I
> if it is possible to move
its contained objects into a different schema after initial creation
of the extension. The default is <TT
CLASS="LITERAL"
>false</TT
>, i.e. the
extension is not relocatable.
See below for more information.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>schema</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
> 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.
</P
></DD
></DL
></DIV
><P
> In addition to the primary control file
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>extension</I
></TT
>.control</TT
>,
an extension can have secondary control files named in the style
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>extension</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>version</I
></TT
>.control</TT
>.
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 <TT
CLASS="VARNAME"
>directory</TT
> and
<TT
CLASS="VARNAME"
>default_version</TT
> cannot be set in a secondary control file.
</P
><P
> An extension's <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> script files can contain any SQL commands,
except for transaction control commands (<TT
CLASS="COMMAND"
>BEGIN</TT
>,
<TT
CLASS="COMMAND"
>COMMIT</TT
>, etc) and commands that cannot be executed inside a
transaction block (such as <TT
CLASS="COMMAND"
>VACUUM</TT
>). This is because the
script files are implicitly executed within a transaction block.
</P
><P
> An extension's <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> script files can also contain lines
beginning with <TT
CLASS="LITERAL"
>\echo</TT
>, which will be ignored (treated as
comments) by the extension mechanism. This provision is commonly used
to throw an error if the script file is fed to <SPAN
CLASS="APPLICATION"
>psql</SPAN
>
rather than being loaded via <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> (see example
script below). Without that, users might accidentally load the
extension's contents as <SPAN
CLASS="QUOTE"
>"loose"</SPAN
> objects rather than as an
extension, a state of affairs that's a bit tedious to recover from.
</P
><P
> 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 <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 <TT
CLASS="VARNAME"
>comment</TT
>
parameter, but instead use <TT
CLASS="COMMAND"
>COMMENT ON EXTENSION</TT
>
within a script file to set the comment.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN54636"
>35.15.2. Extension Relocatability</A
></H2
><P
> 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:
</P
><P
></P
><UL
><LI
><P
> 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 <TT
CLASS="COMMAND"
>ALTER EXTENSION SET SCHEMA</TT
>
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 <TT
CLASS="LITERAL"
>relocatable = true</TT
> in its control
file.
</P
></LI
><LI
><P
> 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 <TT
CLASS="LITERAL"
>search_path</TT
> properties for SQL functions.
For such an extension, set <TT
CLASS="LITERAL"
>relocatable = false</TT
> in its
control file, and use <TT
CLASS="LITERAL"
>@extschema@</TT
> 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
<TT
CLASS="LITERAL"
>SCHEMA</TT
> option of <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
>.
</P
></LI
><LI
><P
> If the extension does not support relocation at all, set
<TT
CLASS="LITERAL"
>relocatable = false</TT
> in its control file, and also set
<TT
CLASS="LITERAL"
>schema</TT
> to the name of the intended target schema. This
will prevent use of the <TT
CLASS="LITERAL"
>SCHEMA</TT
> option of <TT
CLASS="COMMAND"
>CREATE
EXTENSION</TT
>, 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 <TT
CLASS="LITERAL"
>@extschema@</TT
>. The <TT
CLASS="LITERAL"
>@extschema@</TT
>
substitution mechanism is available in this case too, although it is
of limited use since the schema name is determined by the control file.
</P
></LI
></UL
><P
> In all cases, the script file will be executed with
<A
HREF="runtime-config-client.html#GUC-SEARCH-PATH"
>search_path</A
> initially set to point to the target
schema; that is, <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> does the equivalent of
this:
</P><PRE
CLASS="PROGRAMLISTING"
>SET LOCAL search_path TO @extschema@;</PRE
><P>
This allows the objects created by the script file to go into the target
schema. The script file can change <TT
CLASS="VARNAME"
>search_path</TT
> if it wishes,
but that is generally undesirable. <TT
CLASS="VARNAME"
>search_path</TT
> is restored
to its previous setting upon completion of <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
>.
</P
><P
> The target schema is determined by the <TT
CLASS="VARNAME"
>schema</TT
> parameter in
the control file if that is given, otherwise by the <TT
CLASS="LITERAL"
>SCHEMA</TT
>
option of <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> if that is given, otherwise the
current default object creation schema (the first one in the caller's
<TT
CLASS="VARNAME"
>search_path</TT
>). When the control file <TT
CLASS="VARNAME"
>schema</TT
>
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.
</P
><P
> If any prerequisite extensions are listed in <TT
CLASS="VARNAME"
>requires</TT
>
in the control file, their target schemas are appended to the initial
setting of <TT
CLASS="VARNAME"
>search_path</TT
>. This allows their objects to be
visible to the new extension's script file.
</P
><P
> 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 <TT
CLASS="VARNAME"
>search_path</TT
> during creation of dependent
extensions.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN54677"
>35.15.3. Extension Configuration Tables</A
></H2
><P
> 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
<SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>. 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.
</P
><P
> To solve this problem, an extension's script file can mark a table
it has created as a configuration table, which will cause
<SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> to include the table's contents (not its
definition) in dumps. To do that, call the function
<CODE
CLASS="FUNCTION"
>pg_extension_config_dump(regclass, text)</CODE
> after creating the
table, for example
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE my_config (key text, value text);
SELECT pg_catalog.pg_extension_config_dump('my_config', '');</PRE
><P>
Any number of tables can be marked this way.
</P
><P
> When the second argument of <CODE
CLASS="FUNCTION"
>pg_extension_config_dump</CODE
> is
an empty string, the entire contents of the table are dumped by
<SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>. 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 <CODE
CLASS="FUNCTION"
>pg_extension_config_dump</CODE
> provides
a <TT
CLASS="LITERAL"
>WHERE</TT
> condition that selects the data to be dumped.
For example, you might do
</P><PRE
CLASS="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');</PRE
><P>
and then make sure that <TT
CLASS="STRUCTFIELD"
>standard_entry</TT
> is true only
in the rows created by the extension's script.
</P
><P
> 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.
</P
><P
> You can alter the filter condition associated with a configuration table
by calling <CODE
CLASS="FUNCTION"
>pg_extension_config_dump</CODE
> again. (This would
typically be useful in an extension update script.) The only way to mark
a table as no longer a configuration table is to dissociate it from the
extension with <TT
CLASS="COMMAND"
>ALTER EXTENSION ... DROP TABLE</TT
>.
</P
><P
> Note that foreign key relationships between these tables will dictate the
order in which the tables are dumped out by pg_dump. Specifically, pg_dump
will attempt to dump the referenced-by table before the referencing table.
As the foreign key relationships are set up at CREATE EXTENSION time (prior
to data being loaded into the tables) circular dependencies are not
supported. When circular dependencies exist, the data will still be dumped
out but the dump will not be able to be restored directly and user
intervention will be required.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN54699"
>35.15.4. Extension Updates</A
></H2
><P
> 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
<I
CLASS="FIRSTTERM"
>update scripts</I
> that make the necessary changes to go from
one version to the next. Update scripts have names following the pattern
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>extension</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>oldversion</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>newversion</I
></TT
>.sql</TT
>
(for example, <TT
CLASS="LITERAL"
>foo--1.0--1.1.sql</TT
> contains the commands to modify
version <TT
CLASS="LITERAL"
>1.0</TT
> of extension <TT
CLASS="LITERAL"
>foo</TT
> into version
<TT
CLASS="LITERAL"
>1.1</TT
>).
</P
><P
> Given that a suitable update script is available, the command
<TT
CLASS="COMMAND"
>ALTER EXTENSION UPDATE</TT
> will update an installed extension
to the specified new version. The update script is run in the same
environment that <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> provides for installation
scripts: in particular, <TT
CLASS="VARNAME"
>search_path</TT
> is set up in the same
way, and any new objects created by the script are automatically added
to the extension.
</P
><P
> 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.
</P
><P
> The update mechanism can be used to solve an important special case:
converting a <SPAN
CLASS="QUOTE"
>"loose"</SPAN
> collection of objects into an extension.
Before the extension mechanism was added to
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> (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 <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> 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 <TT
CLASS="COMMAND"
>ALTER
EXTENSION ADD</TT
> 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. <TT
CLASS="COMMAND"
>CREATE
EXTENSION</TT
> supports this case with its <TT
CLASS="LITERAL"
>FROM</TT
> <TT
CLASS="REPLACEABLE"
><I
>old_version</I
></TT
> option, which causes it to not run the
normal installation script for the target version, but instead the update
script named
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>extension</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>old_version</I
></TT
>--<TT
CLASS="REPLACEABLE"
><I
>target_version</I
></TT
>.sql</TT
>.
The choice of the dummy version name to use as <TT
CLASS="REPLACEABLE"
><I
>old_version</I
></TT
> is up to the extension author, though
<TT
CLASS="LITERAL"
>unpackaged</TT
> 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.
</P
><P
> <TT
CLASS="COMMAND"
>ALTER EXTENSION</TT
> is able to execute sequences of update
script files to achieve a requested update. For example, if only
<TT
CLASS="LITERAL"
>foo--1.0--1.1.sql</TT
> and <TT
CLASS="LITERAL"
>foo--1.1--2.0.sql</TT
> are
available, <TT
CLASS="COMMAND"
>ALTER EXTENSION</TT
> will apply them in sequence if an
update to version <TT
CLASS="LITERAL"
>2.0</TT
> is requested when <TT
CLASS="LITERAL"
>1.0</TT
> is
currently installed.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> doesn't assume anything about the properties
of version names: for example, it does not know whether <TT
CLASS="LITERAL"
>1.1</TT
>
follows <TT
CLASS="LITERAL"
>1.0</TT
>. 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
<TT
CLASS="LITERAL"
>--</TT
> or leading or trailing <TT
CLASS="LITERAL"
>-</TT
>.)
</P
><P
> Sometimes it is useful to provide <SPAN
CLASS="QUOTE"
>"downgrade"</SPAN
> scripts, for
example <TT
CLASS="LITERAL"
>foo--1.1--1.0.sql</TT
> to allow reverting the changes
associated with version <TT
CLASS="LITERAL"
>1.1</TT
>. 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 <SPAN
CLASS="QUOTE"
>"fast path"</SPAN
> 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.
</P
><P
> To check for unexpected update paths, use this command:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM pg_extension_update_paths('<TT
CLASS="REPLACEABLE"
><I
>extension_name</I
></TT
>');</PRE
><P>
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 <TT
CLASS="LITERAL"
>NULL</TT
> if
there is no available update path. The path is shown in textual form
with <TT
CLASS="LITERAL"
>--</TT
> separators. You can use
<TT
CLASS="LITERAL"
>regexp_split_to_array(path,'--')</TT
> if you prefer an array
format.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN54754"
>35.15.5. Extension Example</A
></H2
><P
> Here is a complete example of an <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>-only
extension, a two-element composite type that can store any type of value
in its slots, which are named <SPAN
CLASS="QUOTE"
>"k"</SPAN
> and <SPAN
CLASS="QUOTE"
>"v"</SPAN
>. Non-text
values are automatically coerced to text for storage.
</P
><P
> The script file <TT
CLASS="FILENAME"
>pair--1.0.sql</TT
> looks like this:
</P><PRE
CLASS="PROGRAMLISTING"
>-- complain if script is sourced in psql, rather than via CREATE EXTENSION
\echo Use "CREATE EXTENSION pair" to load this file. \quit
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);</PRE
><P>
</P
><P
> The control file <TT
CLASS="FILENAME"
>pair.control</TT
> looks like this:
</P><PRE
CLASS="PROGRAMLISTING"
># pair extension
comment = 'A key/value pair data type'
default_version = '1.0'
relocatable = true</PRE
><P>
</P
><P
> While you hardly need a makefile to install these two files into the
correct directory, you could use a <TT
CLASS="FILENAME"
>Makefile</TT
> containing this:
</P><PRE
CLASS="PROGRAMLISTING"
>EXTENSION = pair
DATA = pair--1.0.sql
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)</PRE
><P>
This makefile relies on <ACRONYM
CLASS="ACRONYM"
>PGXS</ACRONYM
>, which is described
in <A
HREF="extend-pgxs.html"
>Section 35.16</A
>. The command <TT
CLASS="LITERAL"
>make install</TT
>
will install the control and script files into the correct
directory as reported by <SPAN
CLASS="APPLICATION"
>pg_config</SPAN
>.
</P
><P
> Once the files are installed, use the
<A
HREF="sql-createextension.html"
>CREATE EXTENSION</A
> command to load the objects into
any particular database.
</P
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