On Sat, Feb 27, 2016 at 10:37 AM, Álvaro Hernández Tortosa <aht@8kdata.com> wrote: > > > 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. >
I don't know what kind of problem you want to solve, but maybe you should ask to yourself:
1) I need to change some current PostgreSQL behavior?
2) I need to add a new feature do PostgreSQL without change the current behavior?
Writing a C extension you can access a lot of internal code if it's available internally by .h headers. For example, some time ago I'm thinking to write an extension to show more internal information about autovacuum (internal queue, etc... some like pg_stat_autovaccuum) . But nowadays is impossible without change the core because some internal structures are not exposed, so we should define an internal API to expose this kind of information.
So depending what problem you want to solve you can write an extension to do that. Then unfortunately the short aswer is "depend".
