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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