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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>38.17. Packaging Related Objects into an Extension</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="xindex.html" title="38.16. Interfacing Extensions to Indexes" /><link rel="next" href="extend-pgxs.html" title="38.18. Extension Building Infrastructure" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">38.17. Packaging Related Objects into an Extension</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="xindex.html" title="38.16. Interfacing Extensions to Indexes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="extend.html" title="Chapter 38. Extending SQL">Up</a></td><th width="60%" align="center">Chapter 38. Extending <acronym class="acronym">SQL</acronym></th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="extend-pgxs.html" title="38.18. Extension Building Infrastructure">Next</a></td></tr></table><hr /></div><div class="sect1" id="EXTEND-EXTENSIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">38.17. Packaging Related Objects into an Extension</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="extend-extensions.html#id-1.8.3.20.11">38.17.1. Extension Files</a></span></dt><dt><span class="sect2"><a href="extend-extensions.html#EXTEND-EXTENSIONS-RELOCATION">38.17.2. Extension Relocatability</a></span></dt><dt><span class="sect2"><a href="extend-extensions.html#EXTEND-EXTENSIONS-CONFIG-TABLES">38.17.3. Extension Configuration Tables</a></span></dt><dt><span class="sect2"><a href="extend-extensions.html#id-1.8.3.20.14">38.17.4. Extension Updates</a></span></dt><dt><span class="sect2"><a href="extend-extensions.html#id-1.8.3.20.15">38.17.5. Installing Extensions Using Update Scripts</a></span></dt><dt><span class="sect2"><a href="extend-extensions.html#EXTEND-EXTENSIONS-SECURITY">38.17.6. Security Considerations for Extensions</a></span></dt><dt><span class="sect2"><a href="extend-extensions.html#EXTEND-EXTENSIONS-EXAMPLE">38.17.7. Extension Example</a></span></dt></dl></div><a id="id-1.8.3.20.2" class="indexterm"></a><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 <em class="firstterm">extension</em>. To define an extension,
you need at least a <em class="firstterm">script file</em> that contains the
<acronym class="acronym">SQL</acronym> commands to create the extension's objects, and a
<em class="firstterm">control file</em> 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 class="link" href="sql-createextension.html" title="CREATE EXTENSION"><code class="command">CREATE EXTENSION</code></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">“<span class="quote">loose</span>”</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 class="link" href="sql-dropextension.html" title="DROP EXTENSION"><code class="command">DROP EXTENSION</code></a>
command (no need to maintain a separate <span class="quote">“<span class="quote">uninstall</span>”</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 <code class="command">CREATE EXTENSION</code> 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 <code class="command">CREATE OR REPLACE FUNCTION</code> 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 <a class="xref" href="extend-extensions.html#EXTEND-EXTENSIONS-CONFIG-TABLES" title="38.17.3. Extension Configuration Tables">Section 38.17.3</a>.)
In production situations, it's generally better to create an extension
update script to perform changes to extension member objects.
</p><p>
The extension script may set privileges on objects that are part of the
extension, using <code class="command">GRANT</code> and <code class="command">REVOKE</code>
statements. The final set of privileges for each object (if any are set)
will be stored in the
<a class="link" href="catalog-pg-init-privs.html" title="53.28. pg_init_privs"><code class="structname">pg_init_privs</code></a>
system catalog. When <span class="application">pg_dump</span> is used, the
<code class="command">CREATE EXTENSION</code> command will be included in the dump, followed
by the set of <code class="command">GRANT</code> and <code class="command">REVOKE</code>
statements necessary to set the privileges on the objects to what they were
at the time the dump was taken.
</p><p>
<span class="productname">PostgreSQL</span> does not currently support extension scripts
issuing <code class="command">CREATE POLICY</code> or <code class="command">SECURITY LABEL</code>
statements. These are expected to be set after the extension has been
created. All RLS policies and security labels on extension objects will be
included in dumps created by <span class="application">pg_dump</span>.
</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 <em class="firstterm">update script</em> that makes just those
two changes. The <code class="command">ALTER EXTENSION UPDATE</code> 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 class="link" href="sql-alterextension.html" title="ALTER EXTENSION"><code class="command">ALTER EXTENSION</code></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">“<span class="quote">within</span>”</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><p>
If an extension's script creates any temporary objects (such as temp
tables), those objects are treated as extension members for the
remainder of the current session, but are automatically dropped at
session end, as any temporary object would be. This is an exception
to the rule that extension member objects cannot be dropped without
dropping the whole extension.
</p><div class="sect2" id="id-1.8.3.20.11"><div class="titlepage"><div><div><h3 class="title">38.17.1. Extension Files</h3></div></div></div><a id="id-1.8.3.20.11.2" class="indexterm"></a><p>
The <code class="command">CREATE EXTENSION</code> command relies on a control
file for each extension, which must be named the same as the extension
with a suffix of <code class="literal">.control</code>, and must be placed in the
installation's <code class="literal">SHAREDIR/extension</code> directory. There
must also be at least one <acronym class="acronym">SQL</acronym> script file, which follows the
naming pattern
<code class="literal"><em class="replaceable"><code>extension</code></em>--<em class="replaceable"><code>version</code></em>.sql</code>
(for example, <code class="literal">foo--1.0.sql</code> for version <code class="literal">1.0</code> of
extension <code class="literal">foo</code>). By default, the script file(s) are also
placed in the <code class="literal">SHAREDIR/extension</code> 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
<code class="filename">postgresql.conf</code> file, namely a list of
<em class="replaceable"><code>parameter_name</code></em> <code class="literal">=</code> <em class="replaceable"><code>value</code></em>
assignments, one per line. Blank lines and comments introduced by
<code class="literal">#</code> 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><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="varname">directory</code> (<code class="type">string</code>)</span></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 <code class="literal">SHAREDIR</code> directory. The
default behavior is equivalent to specifying
<code class="literal">directory = 'extension'</code>.
</p></dd><dt><span class="term"><code class="varname">default_version</code> (<code class="type">string</code>)</span></dt><dd><p>
The default version of the extension (the one that will be installed
if no version is specified in <code class="command">CREATE EXTENSION</code>). Although
this can be omitted, that will result in <code class="command">CREATE EXTENSION</code>
failing if no <code class="literal">VERSION</code> option appears, so you generally
don't want to do that.
</p></dd><dt><span class="term"><code class="varname">comment</code> (<code class="type">string</code>)</span></dt><dd><p>
A comment (any string) about the extension. The comment is applied
when initially creating an extension, but not during extension updates
(since that might override user-added comments). Alternatively,
the extension's comment can be set by writing
a <a class="xref" href="sql-comment.html" title="COMMENT"><span class="refentrytitle">COMMENT</span></a> command in the script file.
</p></dd><dt><span class="term"><code class="varname">encoding</code> (<code class="type">string</code>)</span></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><span class="term"><code class="varname">module_pathname</code> (<code class="type">string</code>)</span></dt><dd><p>
The value of this parameter will be substituted for each occurrence
of <code class="literal">MODULE_PATHNAME</code> in the script file(s). If it is not
set, no substitution is made. Typically, this is set to
<code class="literal">$libdir/<em class="replaceable"><code>shared_library_name</code></em></code> and
then <code class="literal">MODULE_PATHNAME</code> is used in <code class="command">CREATE
FUNCTION</code> 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><span class="term"><code class="varname">requires</code> (<code class="type">string</code>)</span></dt><dd><p>
A list of names of extensions that this extension depends on,
for example <code class="literal">requires = 'foo, bar'</code>. Those
extensions must be installed before this one can be installed.
</p></dd><dt><span class="term"><code class="varname">superuser</code> (<code class="type">boolean</code>)</span></dt><dd><p>
If this parameter is <code class="literal">true</code> (which is the default),
only superusers can create the extension or update it to a new
version (but see also <code class="varname">trusted</code>, below).
If it is set to <code class="literal">false</code>, just the privileges
required to execute the commands in the installation or update script
are required.
This should normally be set to <code class="literal">true</code> if any of the
script commands require superuser privileges. (Such commands would
fail anyway, but it's more user-friendly to give the error up front.)
</p></dd><dt><span class="term"><code class="varname">trusted</code> (<code class="type">boolean</code>)</span></dt><dd><p>
This parameter, if set to <code class="literal">true</code> (which is not the
default), allows some non-superusers to install an extension that
has <code class="varname">superuser</code> set to <code class="literal">true</code>.
Specifically, installation will be permitted for anyone who has
<code class="literal">CREATE</code> privilege on the current database.
When the user executing <code class="command">CREATE EXTENSION</code> is not
a superuser but is allowed to install by virtue of this parameter,
then the installation or update script is run as the bootstrap
superuser, not as the calling user.
This parameter is irrelevant if <code class="varname">superuser</code> is
<code class="literal">false</code>.
Generally, this should not be set true for extensions that could
allow access to otherwise-superuser-only abilities, such as
file system access.
Also, marking an extension trusted requires significant extra effort
to write the extension's installation and update script(s) securely;
see <a class="xref" href="extend-extensions.html#EXTEND-EXTENSIONS-SECURITY" title="38.17.6. Security Considerations for Extensions">Section 38.17.6</a>.
</p></dd><dt><span class="term"><code class="varname">relocatable</code> (<code class="type">boolean</code>)</span></dt><dd><p>
An extension is <em class="firstterm">relocatable</em> if it is possible to move
its contained objects into a different schema after initial creation
of the extension. The default is <code class="literal">false</code>, i.e., the
extension is not relocatable.
See <a class="xref" href="extend-extensions.html#EXTEND-EXTENSIONS-RELOCATION" title="38.17.2. Extension Relocatability">Section 38.17.2</a> for more information.
</p></dd><dt><span class="term"><code class="varname">schema</code> (<code class="type">string</code>)</span></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.
The <code class="varname">schema</code> parameter is consulted only when
initially creating an extension, not during extension updates.
See <a class="xref" href="extend-extensions.html#EXTEND-EXTENSIONS-RELOCATION" title="38.17.2. Extension Relocatability">Section 38.17.2</a> for more information.
</p></dd></dl></div><p>
In addition to the primary control file
<code class="literal"><em class="replaceable"><code>extension</code></em>.control</code>,
an extension can have secondary control files named in the style
<code class="literal"><em class="replaceable"><code>extension</code></em>--<em class="replaceable"><code>version</code></em>.control</code>.
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 <code class="varname">directory</code> and
<code class="varname">default_version</code> 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 (<code class="command">BEGIN</code>,
<code class="command">COMMIT</code>, etc.) and commands that cannot be executed inside a
transaction block (such as <code class="command">VACUUM</code>). 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 <code class="literal">\echo</code>, 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 <code class="command">CREATE EXTENSION</code> (see example
script in <a class="xref" href="extend-extensions.html#EXTEND-EXTENSIONS-EXAMPLE" title="38.17.7. Extension Example">Section 38.17.7</a>).
Without that, users might accidentally load the
extension's contents as <span class="quote">“<span class="quote">loose</span>”</span> objects rather than as an
extension, a state of affairs that's a bit tedious to recover from.
</p><p>
If the extension script contains the
string <code class="literal">@extowner@</code>, that string is replaced with the
(suitably quoted) name of the user calling <code class="command">CREATE
EXTENSION</code> or <code class="command">ALTER EXTENSION</code>. Typically
this feature is used by extensions that are marked trusted to assign
ownership of selected objects to the calling user rather than the
bootstrap superuser. (One should be careful about doing so, however.
For example, assigning ownership of a C-language function to a
non-superuser would create a privilege escalation path for that user.)
</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 <code class="varname">comment</code>
parameter, but instead use <code class="command">COMMENT ON EXTENSION</code>
within a script file to set the comment.
</p></div><div class="sect2" id="EXTEND-EXTENSIONS-RELOCATION"><div class="titlepage"><div><div><h3 class="title">38.17.2. Extension Relocatability</h3></div></div></div><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><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><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 <code class="command">ALTER EXTENSION SET SCHEMA</code>
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 <code class="literal">relocatable = true</code> in its control
file.
</p></li><li class="listitem"><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 <code class="literal">search_path</code> properties for SQL functions.
For such an extension, set <code class="literal">relocatable = false</code> in its
control file, and use <code class="literal">@extschema@</code> 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
<code class="literal">SCHEMA</code> option of <code class="command">CREATE EXTENSION</code>.
</p></li><li class="listitem"><p>
If the extension does not support relocation at all, set
<code class="literal">relocatable = false</code> in its control file, and also set
<code class="literal">schema</code> to the name of the intended target schema. This
will prevent use of the <code class="literal">SCHEMA</code> option of <code class="command">CREATE
EXTENSION</code>, 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 <code class="literal">@extschema@</code>. The <code class="literal">@extschema@</code>
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></div><p>
In all cases, the script file will be executed with
<a class="xref" href="runtime-config-client.html#GUC-SEARCH-PATH">search_path</a> initially set to point to the target
schema; that is, <code class="command">CREATE EXTENSION</code> does the equivalent of
this:
</p><pre class="programlisting">
SET LOCAL search_path TO @extschema@, pg_temp;
</pre><p>
This allows the objects created by the script file to go into the target
schema. The script file can change <code class="varname">search_path</code> if it wishes,
but that is generally undesirable. <code class="varname">search_path</code> is restored
to its previous setting upon completion of <code class="command">CREATE EXTENSION</code>.
</p><p>
The target schema is determined by the <code class="varname">schema</code> parameter in
the control file if that is given, otherwise by the <code class="literal">SCHEMA</code>
option of <code class="command">CREATE EXTENSION</code> if that is given, otherwise the
current default object creation schema (the first one in the caller's
<code class="varname">search_path</code>). When the control file <code class="varname">schema</code>
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 <code class="varname">requires</code>
in the control file, their target schemas are added to the initial
setting of <code class="varname">search_path</code>, following the new
extension's target schema. This allows their objects to be visible to
the new extension's script file.
</p><p>
For security, <code class="literal">pg_temp</code> is automatically appended to
the end of <code class="varname">search_path</code> in all cases.
</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 <code class="varname">search_path</code> during creation of dependent
extensions.
</p></div><div class="sect2" id="EXTEND-EXTENSIONS-CONFIG-TABLES"><div class="titlepage"><div><div><h3 class="title">38.17.3. Extension Configuration Tables</h3></div></div></div><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 restore.
</p><a id="id-1.8.3.20.13.3" class="indexterm"></a><p>
To solve this problem, an extension's script file can mark a table
or a sequence it has created as a configuration relation, which will
cause <span class="application">pg_dump</span> to include the table's or the sequence'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 or the sequence, for example
</p><pre class="programlisting">
CREATE TABLE my_config (key text, value text);
CREATE SEQUENCE my_config_seq;
SELECT pg_catalog.pg_extension_config_dump('my_config', '');
SELECT pg_catalog.pg_extension_config_dump('my_config_seq', '');
</pre><p>
Any number of tables or sequences can be marked this way. Sequences
associated with <code class="type">serial</code> or <code class="type">bigserial</code> columns can
be marked as well.
</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 <code class="literal">WHERE</code> 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 <code class="structfield">standard_entry</code> is true only
in the rows created by the extension's script.
</p><p>
For sequences, the second argument of <code class="function">pg_extension_config_dump</code>
has no effect.
</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 <code class="command">ALTER EXTENSION ... DROP TABLE</code>.
</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><p>
Sequences associated with <code class="type">serial</code> or <code class="type">bigserial</code> columns
need to be directly marked to dump their state. Marking their parent
relation is not enough for this purpose.
</p></div><div class="sect2" id="id-1.8.3.20.14"><div class="titlepage"><div><div><h3 class="title">38.17.4. Extension Updates</h3></div></div></div><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
<em class="firstterm">update scripts</em> that make the necessary changes to go from
one version to the next. Update scripts have names following the pattern
<code class="literal"><em class="replaceable"><code>extension</code></em>--<em class="replaceable"><code>old_version</code></em>--<em class="replaceable"><code>target_version</code></em>.sql</code>
(for example, <code class="literal">foo--1.0--1.1.sql</code> contains the commands to modify
version <code class="literal">1.0</code> of extension <code class="literal">foo</code> into version
<code class="literal">1.1</code>).
</p><p>
Given that a suitable update script is available, the command
<code class="command">ALTER EXTENSION UPDATE</code> will update an installed extension
to the specified new version. The update script is run in the same
environment that <code class="command">CREATE EXTENSION</code> provides for installation
scripts: in particular, <code class="varname">search_path</code> is set up in the same
way, and any new objects created by the script are automatically added
to the extension. Also, if the script chooses to drop extension member
objects, they are automatically dissociated from 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>
<code class="command">ALTER EXTENSION</code> is able to execute sequences of update
script files to achieve a requested update. For example, if only
<code class="literal">foo--1.0--1.1.sql</code> and <code class="literal">foo--1.1--2.0.sql</code> are
available, <code class="command">ALTER EXTENSION</code> will apply them in sequence if an
update to version <code class="literal">2.0</code> is requested when <code class="literal">1.0</code> 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 <code class="literal">1.1</code>
follows <code class="literal">1.0</code>. 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
<code class="literal">--</code> or leading or trailing <code class="literal">-</code>.)
</p><p>
Sometimes it is useful to provide <span class="quote">“<span class="quote">downgrade</span>”</span> scripts, for
example <code class="literal">foo--1.1--1.0.sql</code> to allow reverting the changes
associated with version <code class="literal">1.1</code>. 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">“<span class="quote">fast path</span>”</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('<em class="replaceable"><code>extension_name</code></em>');
</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 <code class="literal">NULL</code> if
there is no available update path. The path is shown in textual form
with <code class="literal">--</code> separators. You can use
<code class="literal">regexp_split_to_array(path,'--')</code> if you prefer an array
format.
</p></div><div class="sect2" id="id-1.8.3.20.15"><div class="titlepage"><div><div><h3 class="title">38.17.5. Installing Extensions Using Update Scripts</h3></div></div></div><p>
An extension that has been around for awhile will probably exist in
several versions, for which the author will need to write update scripts.
For example, if you have released a <code class="literal">foo</code> extension in
versions <code class="literal">1.0</code>, <code class="literal">1.1</code>, and <code class="literal">1.2</code>, there
should be update scripts <code class="filename">foo--1.0--1.1.sql</code>
and <code class="filename">foo--1.1--1.2.sql</code>.
Before <span class="productname">PostgreSQL</span> 10, it was necessary to also create
new script files <code class="filename">foo--1.1.sql</code> and <code class="filename">foo--1.2.sql</code>
that directly build the newer extension versions, or else the newer
versions could not be installed directly, only by
installing <code class="literal">1.0</code> and then updating. That was tedious and
duplicative, but now it's unnecessary, because <code class="command">CREATE
EXTENSION</code> can follow update chains automatically.
For example, if only the script
files <code class="filename">foo--1.0.sql</code>, <code class="filename">foo--1.0--1.1.sql</code>,
and <code class="filename">foo--1.1--1.2.sql</code> are available then a request to
install version <code class="literal">1.2</code> is honored by running those three
scripts in sequence. The processing is the same as if you'd first
installed <code class="literal">1.0</code> and then updated to <code class="literal">1.2</code>.
(As with <code class="command">ALTER EXTENSION UPDATE</code>, if multiple pathways are
available then the shortest is preferred.) Arranging an extension's
script files in this style can reduce the amount of maintenance effort
needed to produce small updates.
</p><p>
If you use secondary (version-specific) control files with an extension
maintained in this style, keep in mind that each version needs a control
file even if it has no stand-alone installation script, as that control
file will determine how the implicit update to that version is performed.
For example, if <code class="filename">foo--1.0.control</code> specifies <code class="literal">requires
= 'bar'</code> but <code class="literal">foo</code>'s other control files do not, the
extension's dependency on <code class="literal">bar</code> will be dropped when updating
from <code class="literal">1.0</code> to another version.
</p></div><div class="sect2" id="EXTEND-EXTENSIONS-SECURITY"><div class="titlepage"><div><div><h3 class="title">38.17.6. Security Considerations for Extensions</h3></div></div></div><p>
Widely-distributed extensions should assume little about the database
they occupy. Therefore, it's appropriate to write functions provided
by an extension in a secure style that cannot be compromised by
search-path-based attacks.
</p><p>
An extension that has the <code class="varname">superuser</code> property set to
true must also consider security hazards for the actions taken within
its installation and update scripts. It is not terribly difficult for
a malicious user to create trojan-horse objects that will compromise
later execution of a carelessly-written extension script, allowing that
user to acquire superuser privileges.
</p><p>
If an extension is marked <code class="varname">trusted</code>, then its
installation schema can be selected by the installing user, who might
intentionally use an insecure schema in hopes of gaining superuser
privileges. Therefore, a trusted extension is extremely exposed from a
security standpoint, and all its script commands must be carefully
examined to ensure that no compromise is possible.
</p><p>
Advice about writing functions securely is provided in
<a class="xref" href="extend-extensions.html#EXTEND-EXTENSIONS-SECURITY-FUNCS" title="38.17.6.1. Security Considerations for Extension Functions">Section 38.17.6.1</a> below, and advice
about writing installation scripts securely is provided in
<a class="xref" href="extend-extensions.html#EXTEND-EXTENSIONS-SECURITY-SCRIPTS" title="38.17.6.2. Security Considerations for Extension Scripts">Section 38.17.6.2</a>.
</p><div class="sect3" id="EXTEND-EXTENSIONS-SECURITY-FUNCS"><div class="titlepage"><div><div><h4 class="title">38.17.6.1. Security Considerations for Extension Functions</h4></div></div></div><p>
SQL-language and PL-language functions provided by extensions are at
risk of search-path-based attacks when they are executed, since
parsing of these functions occurs at execution time not creation time.
</p><p>
The <a class="link" href="sql-createfunction.html#SQL-CREATEFUNCTION-SECURITY" title="Writing SECURITY DEFINER Functions Safely"><code class="command">CREATE
FUNCTION</code></a> reference page contains advice about
writing <code class="literal">SECURITY DEFINER</code> functions safely. It's
good practice to apply those techniques for any function provided by
an extension, since the function might be called by a high-privilege
user.
</p><p>
If you cannot set the <code class="varname">search_path</code> to contain only
secure schemas, assume that each unqualified name could resolve to an
object that a malicious user has defined. Beware of constructs that
depend on <code class="varname">search_path</code> implicitly; for
example, <code class="token">IN</code>
and <code class="literal">CASE <em class="replaceable"><code>expression</code></em> WHEN</code>
always select an operator using the search path. In their place, use
<code class="literal">OPERATOR(<em class="replaceable"><code>schema</code></em>.=) ANY</code>
and <code class="literal">CASE WHEN <em class="replaceable"><code>expression</code></em></code>.
</p><p>
A general-purpose extension usually should not assume that it's been
installed into a secure schema, which means that even schema-qualified
references to its own objects are not entirely risk-free. For
example, if the extension has defined a
function <code class="literal">myschema.myfunc(bigint)</code> then a call such
as <code class="literal">myschema.myfunc(42)</code> could be captured by a
hostile function <code class="literal">myschema.myfunc(integer)</code>. Be
careful that the data types of function and operator parameters exactly
match the declared argument types, using explicit casts where necessary.
</p></div><div class="sect3" id="EXTEND-EXTENSIONS-SECURITY-SCRIPTS"><div class="titlepage"><div><div><h4 class="title">38.17.6.2. Security Considerations for Extension Scripts</h4></div></div></div><p>
An extension installation or update script should be written to guard
against search-path-based attacks occurring when the script executes.
If an object reference in the script can be made to resolve to some
other object than the script author intended, then a compromise might
occur immediately, or later when the mis-defined extension object is
used.
</p><p>
DDL commands such as <code class="command">CREATE FUNCTION</code>
and <code class="command">CREATE OPERATOR CLASS</code> are generally secure,
but beware of any command having a general-purpose expression as a
component. For example, <code class="command">CREATE VIEW</code> needs to be
vetted, as does a <code class="literal">DEFAULT</code> expression
in <code class="command">CREATE FUNCTION</code>.
</p><p>
Sometimes an extension script might need to execute general-purpose
SQL, for example to make catalog adjustments that aren't possible via
DDL. Be careful to execute such commands with a
secure <code class="varname">search_path</code>; do <span class="emphasis"><em>not</em></span>
trust the path provided by <code class="command">CREATE/ALTER EXTENSION</code>
to be secure. Best practice is to temporarily
set <code class="varname">search_path</code> to <code class="literal">'pg_catalog,
pg_temp'</code> and insert references to the extension's
installation schema explicitly where needed. (This practice might
also be helpful for creating views.) Examples can be found in
the <code class="filename">contrib</code> modules in
the <span class="productname">PostgreSQL</span> source code distribution.
</p><p>
Cross-extension references are extremely difficult to make fully
secure, partially because of uncertainty about which schema the other
extension is in. The hazards are reduced if both extensions are
installed in the same schema, because then a hostile object cannot be
placed ahead of the referenced extension in the installation-time
<code class="varname">search_path</code>. However, no mechanism currently exists
to require that. For now, best practice is to not mark an extension
trusted if it depends on another one, unless that other one is always
installed in <code class="literal">pg_catalog</code>.
</p></div></div><div class="sect2" id="EXTEND-EXTENSIONS-EXAMPLE"><div class="titlepage"><div><div><h3 class="title">38.17.7. Extension Example</h3></div></div></div><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">“<span class="quote">k</span>”</span> and <span class="quote">“<span class="quote">v</span>”</span>. Non-text
values are automatically coerced to text for storage.
</p><p>
The script file <code class="filename">pair--1.0.sql</code> 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 FUNCTION pair(text, text)
RETURNS pair LANGUAGE SQL AS 'SELECT ROW($1, $2)::@extschema@.pair;';
CREATE OPERATOR ~> (LEFTARG = text, RIGHTARG = text, FUNCTION = pair);
-- "SET search_path" is easy to get right, but qualified names perform better.
CREATE FUNCTION lower(pair)
RETURNS pair LANGUAGE SQL
AS 'SELECT ROW(lower($1.k), lower($1.v))::@extschema@.pair;'
SET search_path = pg_temp;
CREATE FUNCTION pair_concat(pair, pair)
RETURNS pair LANGUAGE SQL
AS 'SELECT ROW($1.k OPERATOR(pg_catalog.||) $2.k,
$1.v OPERATOR(pg_catalog.||) $2.v)::@extschema@.pair;';
</pre><p>
</p><p>
The control file <code class="filename">pair.control</code> looks like this:
</p><pre class="programlisting">
# pair extension
comment = 'A key/value pair data type'
default_version = '1.0'
# cannot be relocatable because of use of @extschema@
relocatable = false
</pre><p>
</p><p>
While you hardly need a makefile to install these two files into the
correct directory, you could use a <code class="filename">Makefile</code> 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 class="xref" href="extend-pgxs.html" title="38.18. Extension Building Infrastructure">Section 38.18</a>. The command <code class="literal">make install</code>
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
<code class="command">CREATE EXTENSION</code> command to load the objects into
any particular database.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="xindex.html" title="38.16. Interfacing Extensions to Indexes">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="extend.html" title="Chapter 38. Extending SQL">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="extend-pgxs.html" title="38.18. Extension Building Infrastructure">Next</a></td></tr><tr><td width="40%" align="left" valign="top">38.16. Interfacing Extensions to Indexes </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 38.18. Extension Building Infrastructure</td></tr></table></div></body></html>
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