37.17. Packaging Related Objects into an Extension

37.17. Packaging Related Objects into an Extension
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37.17.1. Extension Files
37.17.2. Extension Relocatability
37.17.3. Extension Configuration Tables
37.17.4. Extension Updates
37.17.5. Installing Extensions Using Update Scripts
37.17.6. Security Considerations for Extensions
37.17.7. Extension Example

+ A useful extension to PostgreSQL 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. PostgreSQL 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 PostgreSQL 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. +

+ PostgreSQL will not let you drop an individual object + contained in an extension, except by dropping the whole extension. + Also, while you can change the definition of an extension member object + (for example, via 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 37.17.3.) + 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. +

+ PostgreSQL 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. +

37.17.1. 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.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 + 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 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 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. +
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 CREATE EXTENSION 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 37.17.6. +
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 37.17.2 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 schema parameter is consulted only when + initially creating an extension, not during extension updates. + See Section 37.17.2 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 37.17.7). + 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 PostgreSQL to know what encoding a + control file is in. In practice this is only an issue if you want to + use non-ASCII characters in the extension's comment. Recommended + practice in that case is to not use the control file comment + parameter, but instead use COMMENT ON EXTENSION + within a script file to set the comment. +

37.17.2. 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 SCHEMA + command, which automatically renames all the member objects into + the new schema. Normally, this is only possible if the extension + contains no internal assumptions about what schema any of its + objects are in. Also, the extension's objects must all be in one + schema to begin with (ignoring objects that do not belong to any + schema, such as procedural languages). Mark a fully relocatable + extension by setting 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 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 schema names 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. +

37.17.3. 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 reload. +

+ 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. +

37.17.4. 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.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. +

+ PostgreSQL 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. +

37.17.5. 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. +

37.17.6. 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 37.17.6.1 below, and advice + about writing installation scripts securely is provided in + Section 37.17.6.2. +

37.17.6.1. 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 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. +

37.17.6.2. 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 PostgreSQL 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. +

+ Do not use CREATE OR REPLACE + FUNCTION, except in an update script that must change the + definition of a function that is known to be an extension member + already. (Likewise for other OR REPLACE options.) + Using OR REPLACE unnecessarily not only has a risk + of accidentally overwriting someone else's function, but it creates a + security hazard since the overwritten function would still be owned by + its original owner, who could modify it. +

37.17.7. 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 37.18. 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. +

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