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Extensions in PostgreSQL are powerful tools that allow users to extend the functionality of the database system. They can range from simple SQL objects to complex binary libraries, providing extra features and capabilities that aren't available in the core PostgreSQL distribution.
Extensions
To define an extension, it's at least required one script file, that contains the SQL commands to create the objects distributed by the extension, and one control file, that specifies a few basic properties of the extension itself.
When extensions are created, installed, or loaded in a database, they deploy a set of bundle objects that aim to extend the functionality of the engine. Those objects can be functions, operators, roles, data types, access methods, and other database object types.
When extensions are dropped, uninstalled, or unloaded from a database, all the objects that created by the extension are removed. An exception to that case is when there are other objects in the database with dependencies on any of the objects defined by the extension.
The implementation of the functionality provided by those objects distributed by the extension can be written in SQL or PL/pgSQL. But it can also be implemented in a separate shared library (binary) file, which is the result of compiling the source code (typically written in C or Rust) that implements the functionality.
In PostgreSQL, extensions are managed through the CREATE EXTENSION, ALTER EXTENSION, DROP EXTENSION, and COMMENT ON EXTENSION commands.
CREATE EXTENSIONcreates, installs, or loads an extension into the database in which the command is executed.ALTER EXTENSIONupdates the extension to a newer version.DROP EXTENSIONdrops, uninstalls, or unloads an extension from the database in which the command is executed.COMMENT ON EXTENSIONstores a comment about the extension as a database object.
When the server is started, it defines an area of memory to which all backend processes can access, to cooperatively run any workloads. In PostgreSQL jargon, that area of memory is referred to as shared memory.
Some extensions that implement functionality using shared libraries, require accessing that shared memory from the code built in those libraries. Those extensions have one more requirement, which is that their shared library files must be load by the main engine process, as soon as the server starts. For those libraries, you need to follow the instructions in load libraries.