Function as parameter to another function in Postgres

Question

Can I create a user defined function in Postgres either through the C-Language Function API or by using pl/pgsql which accepts a callback function as parameter?

As far as I see there is no way to do this through the C-Language API since it only accepts sql datatypes and there is no datatype for function. But maybe I'm missing something?

Answer 1

Since each function / procedure must have an entry in pg_proc, you can use the primary key for identifying the procedure. This would also eliminate the problems with procedures having the same name but different number of parameters or different parameter types.

Shorthands for this are the types regproc and regprocedure with the associated casts for easier handling. Lookup the manual for these.

Identifying the function and passing it around is no problem:

select 'pg_database_size(oid)'::regprocedure; -- create "reference"
     regprocedure      
-----------------------
 pg_database_size(oid)

Use regprocedure as the parameter type.

The problem I did not yet figure out is how to actually call such a thing in a convenient way.