Re: Curing plpgsql's memory leaks for statement-lifespan values

Hi

2016-07-27 16:49 GMT+02:00 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:

> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Mon, Jul 25, 2016 at 6:04 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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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