PostgreSQL extension API? Documentation?

Hi.

I have a newbie question for extension development. Extensions
provide an entry point, and are dynamically linked to PostgreSQL. But
what APIs/functions are really available for extensions to call?

The most obvious API is SPI. You could also implements hooks. Of
course, functions, types, aggregates, whatever. But can an extension
call other "internal" PostgreSQL functions? Is there any restriction to
what could --or should-- call an extension? Is there any specific API,
or any documentation which states what is available to use?

In other words: what is the API surface exposed by PostgreSQL to
extension developers? The assumption is that no PostgreSQL code should
be modified, just adding your own and calling existing funcitons.

Thanks,

Álvaro

--
Álvaro Hernández Tortosa

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