35.16. Packaging Related Objects into an Extension#
35.16. Packaging Related Objects into an Extension #
A useful extension to Tantor SE-1C 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. Tantor SE-1C calls
such a package an extension. To define an extension,
you need at least a script file that contains the
SQL commands to create the extension's objects, and a
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
CREATE EXTENSION command loads the objects into
your database.
The main advantage of using an extension, rather than just running the
SQL script to load a bunch of “loose” objects
into your database, is that Tantor SE-1C will then
understand that the objects of the extension go together. You can
drop all the objects with a single DROP EXTENSION
command (no need to maintain a separate “uninstall” script).
Even more useful, pg_dump knows that it should not
dump the individual member objects of the extension — it will
just include a 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.
Tantor SE-1C 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 CREATE OR REPLACE FUNCTION for a
function), bear in mind that the modified definition will not be dumped
by 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 Section 35.16.3.)
In production situations, it's generally better to create an extension
update script to perform changes to extension member objects.
The extension script may set privileges on objects that are part of the
extension, using GRANT and REVOKE
statements. The final set of privileges for each object (if any are set)
will be stored in the
pg_init_privs
system catalog. When pg_dump is used, the
CREATE EXTENSION command will be included in the dump, followed
by the set of GRANT and REVOKE
statements necessary to set the privileges on the objects to what they were
at the time the dump was taken.
Tantor SE-1C does not currently support extension scripts
issuing CREATE POLICY or SECURITY LABEL
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 pg_dump.
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 update script that makes just those
two changes. The 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.
The kinds of SQL objects that can be members of an extension are shown in
the description of ALTER EXTENSION. 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 “within” 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.
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.
35.16.1. Extension Files #
The CREATE EXTENSION command relies on a control
file for each extension, which must be named the same as the extension
with a suffix of .control, and must be placed in the
installation's SHAREDIR/extension directory. There
must also be at least one SQL script file, which follows the
naming pattern
extension--version.sqlfoo--1.0.sql for version 1.0 of
extension foo). By default, the script file(s) are also
placed in the SHAREDIR/extension directory; but the
control file can specify a different directory for the script file(s).
The file format for an extension control file is the same as for the
postgresql.conf file, namely a list of
parameter_name = value
assignments, one per line. Blank lines and comments introduced by
# are allowed. Be sure to quote any value that is not
a single word or number.
A control file can set the following parameters:
directory(string) #The directory containing the extension's SQL script file(s). Unless an absolute path is given, the name is relative to the installation's
SHAREDIRdirectory. The default behavior is equivalent to specifyingdirectory = 'extension'.default_version(string) #The default version of the extension (the one that will be installed if no version is specified in
CREATE EXTENSION). Although this can be omitted, that will result inCREATE EXTENSIONfailing if noVERSIONoption appears, so you generally don't want to do that.comment(string) #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 COMMENT command in the script file.
encoding(string) #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.
module_pathname(string) #The value of this parameter will be substituted for each occurrence of
MODULE_PATHNAMEin the script file(s). If it is not set, no substitution is made. Typically, this is set to$libdir/and thenshared_library_nameMODULE_PATHNAMEis used inCREATE FUNCTIONcommands for C-language functions, so that the script files do not need to hard-wire the name of the shared library.requires(string) #A list of names of extensions that this extension depends on, for example
requires = 'foo, bar'. Those extensions must be installed before this one can be installed.no_relocate(string) #A list of names of extensions that this extension depends on that should be barred from changing their schemas via
ALTER EXTENSION ... SET SCHEMA. This is needed if this extension's script references the name of a required extension's schema (using the@extschema:syntax) in a way that cannot track renames.name@superuser(boolean) #If this parameter is
true(which is the default), only superusers can create the extension or update it to a new version (but see alsotrusted, below). If it is set tofalse, just the privileges required to execute the commands in the installation or update script are required. This should normally be set totrueif 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.)trusted(boolean) #This parameter, if set to
true(which is not the default), allows some non-superusers to install an extension that hassuperuserset totrue. Specifically, installation will be permitted for anyone who hasCREATEprivilege on the current database. When the user executingCREATE EXTENSIONis 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 ifsuperuserisfalse. 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 Section 35.16.6.relocatable(boolean) #An extension is relocatable if it is possible to move its contained objects into a different schema after initial creation of the extension. The default is
false, i.e., the extension is not relocatable. See Section 35.16.2 for more information.schema(string) #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
schemaparameter is consulted only when initially creating an extension, not during extension updates. See Section 35.16.2 for more information.
In addition to the primary control file
extension.controlextension--version.controldirectory and
default_version cannot be set in a secondary control file.
An extension's SQL script files can contain any SQL commands,
except for transaction control commands (BEGIN,
COMMIT, etc.) and commands that cannot be executed inside a
transaction block (such as VACUUM). This is because the
script files are implicitly executed within a transaction block.
An extension's SQL script files can also contain lines
beginning with \echo, 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 psql
rather than being loaded via CREATE EXTENSION (see example
script in Section 35.16.7).
Without that, users might accidentally load the
extension's contents as “loose” objects rather than as an
extension, a state of affairs that's a bit tedious to recover from.
If the extension script contains the
string @extowner@, that string is replaced with the
(suitably quoted) name of the user calling CREATE
EXTENSION or ALTER EXTENSION. 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.)
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 Tantor SE-1C 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 comment
parameter, but instead use COMMENT ON EXTENSION
within a script file to set the comment.
35.16.2. Extension Relocatability #
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:
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
ALTER EXTENSION SET SCHEMAcommand, 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 settingrelocatable = truein its control file.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
search_pathproperties for SQL functions. For such an extension, setrelocatable = falsein its control file, and use@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 (double-quoted if necessary) before the script is executed. The user can set the target schema using theSCHEMAoption ofCREATE EXTENSION.If the extension does not support relocation at all, set
relocatable = falsein its control file, and also setschemato the name of the intended target schema. This will prevent use of theSCHEMAoption ofCREATE EXTENSION, unless it specifies the same schema named in the control file. This choice is typically necessary if the extension contains internal assumptions about its schema name that can't be replaced by uses of@extschema@. The@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.
In all cases, the script file will be executed with
search_path initially set to point to the target
schema; that is, CREATE EXTENSION does the equivalent of
this:
SET LOCAL search_path TO @extschema@, pg_temp;
This allows the objects created by the script file to go into the target
schema. The script file can change search_path if it wishes,
but that is generally undesirable. search_path is restored
to its previous setting upon completion of CREATE EXTENSION.
The target schema is determined by the schema parameter in
the control file if that is given, otherwise by the SCHEMA
option of CREATE EXTENSION if that is given, otherwise the
current default object creation schema (the first one in the caller's
search_path). When the control file 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.
If any prerequisite extensions are listed in requires
in the control file, their target schemas are added to the initial
setting of search_path, following the new
extension's target schema. This allows their objects to be visible to
the new extension's script file.
For security, pg_temp is automatically appended to
the end of search_path in all cases.
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 search_path during creation of dependent
extensions.
If an extension references objects belonging to another extension,
it is recommended to schema-qualify those references. To do that,
write @extschema:
in the extension's script file, where name@name
is the name of the other extension (which must be listed in this
extension's requires list). This string will be
replaced by the name (double-quoted if necessary) of that extension's
target schema.
Although this notation avoids the need to make hard-wired assumptions
about schema names in the extension's script file, its use may embed
the other extension's schema name into the installed objects of this
extension. (Typically, that happens
when @extschema: is
used inside a string literal, such as a function body or
a name@search_path setting. In other cases, the object
reference is reduced to an OID during parsing and does not require
subsequent lookups.) If the other extension's schema name is so
embedded, you should prevent the other extension from being relocated
after yours is installed, by adding the name of the other extension to
this one's no_relocate list.
35.16.3. Extension Configuration Tables #
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 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 restore.
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 pg_dump to include the table's or the sequence's
contents (not its definition) in dumps. To do that, call the function
pg_extension_config_dump(regclass, text) after creating the
table or the sequence, for example
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', '');
Any number of tables or sequences can be marked this way. Sequences
associated with serial or bigserial columns can
be marked as well.
When the second argument of pg_extension_config_dump is
an empty string, the entire contents of the table are dumped by
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 pg_extension_config_dump provides
a WHERE condition that selects the data to be dumped.
For example, you might do
CREATE TABLE my_config (key text, value text, standard_entry boolean);
SELECT pg_catalog.pg_extension_config_dump('my_config', 'WHERE NOT standard_entry');
and then make sure that standard_entry is true only
in the rows created by the extension's script.
For sequences, the second argument of pg_extension_config_dump
has no effect.
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.
You can alter the filter condition associated with a configuration table
by calling pg_extension_config_dump 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 ALTER EXTENSION ... DROP TABLE.
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.
Sequences associated with serial or bigserial columns
need to be directly marked to dump their state. Marking their parent
relation is not enough for this purpose.
35.16.4. Extension Updates #
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
update scripts that make the necessary changes to go from
one version to the next. Update scripts have names following the pattern
extension--old_version--target_version.sqlfoo--1.0--1.1.sql contains the commands to modify
version 1.0 of extension foo into version
1.1).
Given that a suitable update script is available, the command
ALTER EXTENSION UPDATE will update an installed extension
to the specified new version. The update script is run in the same
environment that CREATE EXTENSION provides for installation
scripts: in particular, search_path 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.
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.
ALTER EXTENSION is able to execute sequences of update
script files to achieve a requested update. For example, if only
foo--1.0--1.1.sql and foo--1.1--2.0.sql are
available, ALTER EXTENSION will apply them in sequence if an
update to version 2.0 is requested when 1.0 is
currently installed.
Tantor SE-1C doesn't assume anything about the properties
of version names: for example, it does not know whether 1.1
follows 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
-- or leading or trailing -.)
Sometimes it is useful to provide “downgrade” scripts, for
example foo--1.1--1.0.sql to allow reverting the changes
associated with version 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 “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.
To check for unexpected update paths, use this command:
SELECT * FROM pg_extension_update_paths('extension_name');
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 NULL if
there is no available update path. The path is shown in textual form
with -- separators. You can use
regexp_split_to_array(path,'--') if you prefer an array
format.
35.16.5. Installing Extensions Using Update Scripts #
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 foo extension in
versions 1.0, 1.1, and 1.2, there
should be update scripts foo--1.0--1.1.sql
and foo--1.1--1.2.sql.
Before PostgreSQL 10, it was necessary to also create
new script files foo--1.1.sql and foo--1.2.sql
that directly build the newer extension versions, or else the newer
versions could not be installed directly, only by
installing 1.0 and then updating. That was tedious and
duplicative, but now it's unnecessary, because CREATE
EXTENSION can follow update chains automatically.
For example, if only the script
files foo--1.0.sql, foo--1.0--1.1.sql,
and foo--1.1--1.2.sql are available then a request to
install version 1.2 is honored by running those three
scripts in sequence. The processing is the same as if you'd first
installed 1.0 and then updated to 1.2.
(As with ALTER EXTENSION UPDATE, 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.
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 foo--1.0.control specifies requires
= 'bar' but foo's other control files do not, the
extension's dependency on bar will be dropped when updating
from 1.0 to another version.
35.16.6. Security Considerations for Extensions #
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.
An extension that has the superuser 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.
If an extension is marked trusted, 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.
Advice about writing functions securely is provided in Section 35.16.6.1 below, and advice about writing installation scripts securely is provided in Section 35.16.6.2.
35.16.6.1. Security Considerations for Extension Functions #
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.
The CREATE
FUNCTION reference page contains advice about
writing SECURITY DEFINER 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.
If you cannot set the search_path 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 search_path implicitly; for
example, IN
and CASE
always select an operator using the search path. In their place, use
expression WHENOPERATOR(
and schema.=) ANYCASE WHEN .
expression
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 myschema.myfunc(bigint) then a call such
as myschema.myfunc(42) could be captured by a
hostile function myschema.myfunc(integer). Be
careful that the data types of function and operator parameters exactly
match the declared argument types, using explicit casts where necessary.
35.16.6.2. Security Considerations for Extension Scripts #
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.
DDL commands such as CREATE FUNCTION
and CREATE OPERATOR CLASS are generally secure,
but beware of any command having a general-purpose expression as a
component. For example, CREATE VIEW needs to be
vetted, as does a DEFAULT expression
in CREATE FUNCTION.
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 search_path; do not
trust the path provided by CREATE/ALTER EXTENSION
to be secure. Best practice is to temporarily
set search_path to pg_catalog,
pg_temp 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 contrib modules in
the Tantor SE-1C source code distribution.
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
search_path. 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 pg_catalog.
35.16.7. Extension Example #
Here is a complete example of an SQL-only extension, a two-element composite type that can store any type of value in its slots, which are named “k” and “v”. Non-text values are automatically coerced to text for storage.
The script file pair--1.0.sql looks like this:
-- 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;';
The control file pair.control looks like this:
# pair extension comment = 'A key/value pair data type' default_version = '1.0' # cannot be relocatable because of use of @extschema@ relocatable = false
While you hardly need a makefile to install these two files into the
correct directory, you could use a Makefile containing this:
EXTENSION = pair DATA = pair--1.0.sql PG_CONFIG = pg_config PGXS := $(shell $(PG_CONFIG) --pgxs) include $(PGXS)
This makefile relies on PGXS, which is described
in Section 35.17. The command make install
will install the control and script files into the correct
directory as reported by pg_config.
Once the files are installed, use the
CREATE EXTENSION command to load the objects into
any particular database.