PostgreSQL Extensions

PostgreSQL Extensions

Dimitri Fontaine has been working on adding extension packaging functionality to PostgreSQL for a while. His hard work has paid off and PostgreSQL 9.1 will have extensions.

How will extensions work? At a cursory glance, extensions are similar to libraries or DLLs in the programming world. Extensions are packaged units of code that can be installed using the CREATE EXTENSION command. This command will execute a script file that installs any of the extensions objects, including any C code that needs to be built or loaded. If this was the only thing that extensions provided, it would be a nice feature. Interestingly, there’s more.

PostgreSQL’s extensions may consist of any number of database level objects – tables, functions, views, etc. These objects are versioned together as part of a discrete package. While PostgreSQL won’t let you drop any database object that’s part of an extension, you could go ahead and break an extension using CREATE OR REPLACE FUNCTION. Those changes won’t be tracked as part of the extension.

Once our extension has been installed, it’s going to be managed as a versioned chunk. The best way to make changes it to upgrade the extension using code supplied by the extension developer. The extension code lets developers supply a version number, as well as some other metadata. To be fair, the extension code in PostgreSQL requires that extension developers supply a version number; extensions without a version number will fail on creation with some kind of helpful error message about an invalid version name. But, once we’ve created an extension with a version number we get a few niceties.

  1. We can tell which version of an extension in installed in every database on our server by looking in the pg_extension table of each database.
  2. We can perform updates by comparing the current version to update scripts. Admittedly, ALTER EXTENSION looks like it should handle this for us, but it’s still cool.

What else do we get from the new extensions?

Configuration! Extensions have to ways of setting up their configuration: control files and configuration tables. Extension developers can supply limited configuration through a control file. The control file lets an extension developer list things like the name, a description, and any other extensions that are needed for proper functionality.

In addition to the control file, extension specific configuration information through configuration tables. Not only can we supply additional functionality, but it’s possible to make our functionality flexible. Like all important data, it’s important to back up the configuration. To make sure that users’ configuration information is backed up, extension developers can use the pg_extension_config_dump function in their extension installation code to mark extension tables that should be a part of back ups; PostgreSQL will ignore extension tables when pg_dump is run. Interestingly, pg_extension_config_dump takes a second parameter that will be used as a search predicate when pg_dump is run. This can be used to only backup user supplied parameters or work through whatever convoluted scheme you’d like (parameters are only backed up on bank holidays).

We also get extra control about where extensions live. Have you ever installed software from a vendor and been a bit annoyed about where it ends up in your database? Both the CREATE EXTENSION and ALTER EXTENSION commands make it easy to relocate an extension to a new schema. DBAs can opt to install extensions into one schema and move them into another at a later date. We could install a vendor library and move it to a safe schema at another time, for example ALTER EXTENSION magic_vendor_security_audit_tools SET SCHEMA vendor_audit;. ALTER EXTENSION will even update the definition of tables that already exist.

I suspect that the extension mechanism in PostgreSQL 9.1 is going to have far reaching effects for PostgreSQL. The PostgreSQL development team are already moving procedural languages into extensions and bundling additional functionality (formerly living in the contrib folder) into extensions. This addition to PostgreSQL should also make it much easier for software vendors to supply additional functionality for PostgreSQL.

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