PostgreSQL functions returning void

Question

Functions written in PL/pgSQL or SQL can be defined as RETURNS void. I recently stumbled upon an odd difference in the result.

Consider the following demo:

CREATE OR REPLACE FUNCTION f_sql()
  RETURNS void AS
'SELECT NULL::void' -- "do nothing", no special meaning
  LANGUAGE sql;

CREATE OR REPLACE FUNCTION f_plpgsql()
  RETURNS void AS
$$
BEGIN
NULL; -- "do nothing", no special meaning
END;
$$  LANGUAGE plpgsql;

_{The function f_sql() uses the only possible way for a SELECT (as last command) in a SQL function that RETURNS void. I use it just because it is the simplest way for the purposes of this test - any other function, with UPDATE or DELETE for instance, shows the same behavior.}

Now, void is a fictive type. While the plpgsql function seems to return the equivalent of an empty string as type void, effectively ''::void. The sql function seems to return NULL::void.

db=# SELECT f_sql() IS NULL;
 ?column?
----------
 t

db=# SELECT f_sql()::text IS NULL;
 ?column?
----------
 t

db=# SELECT f_plpgsql() IS NULL;
 ?column?
----------
 f

db=# SELECT f_plpgsql()::text = '';
 ?column?
----------
 t

This can have subtle and confusing side effects.
What is the reason behind the difference?

Answer 1

(I'm no expert in this source code. You've been warned.)

The source is online here. I've omitted the filenames; you can search for the function names to find their definitions. I left the line numbers (usually) because it's easier to cut and paste, and different line numbers will mean the source has changed.

The short story is that some "void" returns are probably empty cstrings (empty null-terminated strings), and others are null pointers.

Here are the parts of the source that look relevant.

00228 /*
00229  * void_out     - output routine for pseudo-type VOID.
00230  *
00231  * We allow this so that "SELECT function_returning_void(...)" works.
00232  */
00233 Datum
00234 void_out(PG_FUNCTION_ARGS)
00235 {
00236     PG_RETURN_CSTRING(pstrdup(""));
00237 }

00251 /*
00252  * void_send    - binary output routine for pseudo-type VOID.
00253  *
00254  * We allow this so that "SELECT function_returning_void(...)" works
00255  * even when binary output is requested.
00256  */
00257 Datum
00258 void_send(PG_FUNCTION_ARGS)
00259 {
00260     StringInfoData buf;
00261 
00262     /* send an empty string */
00263     pq_begintypsend(&buf);
00264     PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
00265 }

We also have

00285 /* To return a NULL do this: */
00286 #define PG_RETURN_NULL()  \
00287     do { fcinfo->isnull = true; return (Datum) 0; } while (0)
00288 
00289 /* A few internal functions return void (which is not the same as NULL!) */
00290 #define PG_RETURN_VOID()     return (Datum) 0

So it makes sense to me that a user-defined function that returns through PG_RETURN_VOID() would not test equivalent to one that returns through void_out() or void_send(). I don't yet know why that is, but I have to stop and get some sleep.