pg_stat_statements oddity with track = all

Hi,

Someone raised an interested point recently on pg_stat_kcache extension for
handling nested statements, which also applies to pg_stat_statements.

The root issue is that when pg_stat_statements tracks nested statements,
there's no way to really make sense of the counters, as top level statements
will also account for underlying statements.  Using a naive example:

=# CREATE FUNCTION f1() RETURNS VOID AS $$BEGIN PERFORM pg_sleep(5); END; $$ LANGUAGE plpgsql;
CREATE FUNCTION

=# SELECT pg_stat_statements_reset();
 pg_stat_statements_reset
--------------------------

(1 row)

=# SELECT f1();
 f1
----

(1 row)

=# select sum(total_exec_time) from pg_stat_statements;
     sum
--------------
 10004.403601
(1 row)

It looks like there was 10s total execution time overall all statements, which
doesn't really make sense.

It's of course possible to avoid that using track = top, but tracking all
nested statements is usually quite useful so it could be better to find a way
to better address that problem.

The only idea I have for that is to add a new field to entry key, for instance
is_toplevel.  The immediate cons is obviously that it could amplify quite a lot
the number of entries tracked, so people may need to increase
pg_stat_statements.max to avoid slowdown if that makes them reach frequent
entry eviction.

Should we address the problem, and in that case do you see a better way for
that, or should we just document this behavior?