Re: Inconsistency in plpgsql's error context reports

On Mon, Dec 11, 2017 at 3:52 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Here's a quick hack at that. I guess the main question that needs to be
> asked is whether we're happy with plpgsql getting so much chattier
> (as per all the regression test changes).
>
> If we're not, the alternative that could be considered is to make SPI
> expose some way to suppress its use of a context callback, and change
> plpgsql to invoke that when dealing with an expression. That would
> be rather more invasive code-wise, but would largely eliminate the
> behavioral change as seen by users.
>
> Another angle, if we do keep it like this, is that maybe SPI should
> export _SPI_error_callback so that plpgsql can use it directly,
> rather than having a copy that needs to be kept in sync.

I confess to never having really grokked, even in the pre-patch state,
why we sometimes get an "SQL statement" context line and sometimes
not. However, what strikes me about this is that the SQL statement is
a completely fabricated one that the user never entered. Consider
this bit from the existing regression test output, unpatched:

create function namedparmcursor_test7() returns void as $$
declare
c1 cursor (p1 int, p2 int) for
select * from tenk1 where thousand = p1 and tenthous = p2;
begin
open c1 (p2 := 77, p1 := 42/0);
end $$ language plpgsql;
select namedparmcursor_test7();
ERROR: division by zero
CONTEXT: SQL statement "SELECT 42/0 AS p1, 77 AS p2;"
PL/pgSQL function namedparmcursor_test7() line 6 at OPEN

Quite obviously, nothing like "SELECT 42/0 AS p1, 77 AS p2;" is
present in there anywhere. When people see an SQL statement in the
context, or at least when I see it, my inclination is to go grep for
where that SQL statement is to be found, and to be unhappy when the
answer is nowhere. It would really be a lot better if we could say
something like

CONTEXT: SQL expression 42/0

...but I realize that's probably hard to do. However, the current
situation is that plpgsql.out contains 5 "SQL statement" context
lines, of which only 1 is an SQL statement that actually appears in
the procedure.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company