Packaging Related Objects into an Extension
A useful extension to QHB 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. QHB 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/RUST code, there will typically also be a shared library file into which the C/RUST 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 QHB
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, qhb_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.
QHB 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 qhb_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
Extension Configuration Tables.) 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 qhb_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.
QHB 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 qhb_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.
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.sql (for example, foo--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 qhb.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 SHAREDIR directory.
The default behavior is equivalent to specifying directory = '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 in CREATE EXTENSION failing if no VERSION option 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_PATHNAME in the script file(s). If it is not set, no substitution is
made. Typically, this is set to $libdir/shared_library_name and then
MODULE_PATHNAME is used in CREATE FUNCTION commands for native-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:name@ syntax) in a way that cannot track renames.
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 also trusted, below).
If it is set to false, just the privileges required to execute the commands in
the installation or update script are required. This should normally be set to
true 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.)
trusted (boolean)
This parameter, if set to true (which is not the default), allows some
non-superusers to install an extension that has superuser set to true.
Specifically, installation will be permitted for anyone who has CREATE privilege
on the current database. When the user executing it 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 superuser is false. 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 Security Considerations for Extensions.
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
Extension Relocatability 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
schema parameter is consulted only when initially creating an extension, not
during extension updates. See Section Extension Relocatability for more
information.
In addition to the primary control file extension.control, an extension can have secondary control files named in the style extension--version.control. 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 directory 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
Extension Example). 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 native-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 QHB 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.
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 setting relocatable = true in 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_path properties for SQL functions. For such an extension, set relocatable = false in 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 the SCHEMA option of
CREATE EXTENSION. -
If the extension does not support relocation at all, set relocatable = false in its control file, and also set schema to the name of the intended target schema. This will prevent use of the SCHEMA option of
CREATE EXTENSION, unless it specifies the same schema named in the control file. This choice is typically necessary if the extension contains internal assumptions about 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@;
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:name@
in the extension's script file, where 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:name@ is used inside a string literal, such as a
function body or a 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.
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 qhb_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 qhb_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 qhb_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 qhb_dump. Specifically, qhb_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.
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.sql (for example, foo--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.
QHB 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.
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.
Previously, 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.
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 Security Considerations for Extension Functions below, and advice about writing installation scripts securely is provided in Security Considerations for Extension Scripts.
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 expression WHEN always select an operator using the search path. In their place, use OPERATOR(schema.=) ANY and CASE 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.
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 share/extension
modules in the QHB 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.
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 OR REPLACE 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 OR REPLACE FUNCTION lower(pair)
RETURNS pair LANGUAGE SQL
AS 'SELECT ROW(lower($1.k), lower($1.v))::@extschema@.pair;'
SET search_path = pg_temp;
CREATE OR REPLACE 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 below. 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.