Re: Clarification on when _PG_init() is invoked for extensions

Hi,

Based on documentation and experimentation, I believe the behavior works as
> follows, but I would appreciate
> confirmation or corrections:
>
> 1/ When the extension is listed in shared_preload_libraries: _PG_init()
> is executed before the server
> starts accepting connections, and hooks are initialized for every backend
> process. The downside is that this enables
> hooks globally for all databases in the instance.
>
That sounds correct. shared_preload_libraries are typically used when your
module needs to access shared
memory or register a background worker.

> 3/ When running CREATE EXTENSION: Here the .so is loaded lazily when a
> function defined with MODULE_PATHNAME is
> first executed or defined. In this model, I’m unsure whether I need to
> explicitly run LOAD 'extension' to ensure
> _PG_init() runs for other existing and upcoming backends.
>
CREATE extension does not automatically load or ensure that _PG_init() is
run.
It mainly runs the .sql script in your extension.

> *My use case:*
> I would like the extension to initialize hooks automatically, but only for
> all (existing and upcoming) backends of the specific
> database where the extension is installed - not globally across all
> databases.
>
You might need to add a check in your hook to verify the current database.
However, depending on where the hook runs, performing catalog lookups to
determine the connected database could be costly.

Could you share more details about the problem you’re trying to solve, or
explain why
you want to restrict access to hooks?

Thank you,
Rahila Syed