Re: Curing plpgsql's memory leaks for statement-lifespan values - Mailing list pgsql-hackers

Hi

2016-07-27 16:49 GMT+02:00 Tom Lane <tgl@sss.pgh.pa.us>:

Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Jul 25, 2016 at 6:04 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I suppose that a fix based on putting PG_TRY blocks into all the affected
>> functions might be simple enough that we'd risk back-patching it, but
>> I don't really want to go that way.

> try/catch blocks aren't completely free, either, and PL/pgsql is not
> suffering from a deplorable excess of execution speed.

BTW, just to annotate that a bit: I did some measurements and found out
that on my Linux box, creating/deleting a memory context
(AllocSetContextCreate + MemoryContextDelete) is somewhere around 10x
more expensive than a PG_TRY block.  This means that the PG_TRY approach
would actually be faster for cases involving only a small number of
statements-needing-local-storage within a single plpgsql function
execution.  However, the memory context creation cost is amortized across
repeated executions of a statement, whereas of course PG_TRY won't be.
We can roughly estimate that PG_TRY would lose any time we loop through
the statement in question more than circa ten times.  So I believe the
way I want to do it will win speed-wise in cases where it matters, but
it's not entirely an open-and-shut conclusion.

Anyway, there are enough other reasons not to go the PG_TRY route.

I did some synthetic benchmarks related to plpgsql speed - bubble sort and loop over handling errors and I don't see any slowdown

handling exceptions is little bit faster with your patch

 CREATE OR REPLACE FUNCTION public.loop_test(a integer)
 RETURNS void
 LANGUAGE plpgsql
AS $function$
declare x int;
begin
  for i in 1..a
  loop
    declare s text;
    begin
      s := 'AHOJ';
      x := (random()*1000)::int;
      raise exception '*****';
    exception when others then
      x := 0; --raise notice 'handled';
    end;
  end loop;
end;
$function$

head - 100K loops 640ms, patched 610ms

Regards

Pavel

                        regards, tom lane

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