Hi, On 2017-03-06 19:49:56 +0100, Tomas Vondra wrote: > On 03/06/2017 07:05 PM, Robert Haas wrote: > > On Mon, Mar 6, 2017 at 12:44 PM, Andres Freund <and...@anarazel.de> wrote: > > > On 2017-03-06 12:40:18 -0500, Robert Haas wrote: > > > > On Wed, Mar 1, 2017 at 5:55 PM, Andres Freund <and...@anarazel.de> > > > > wrote: > > > > > The issue was that on 32bit platforms the Datum returned by some > > > > > functions (int2int4_sum in this case) isn't actually a separately > > > > > allocated Datum, but rather just something embedded in a larger > > > > > struct. That, combined with the following code: > > > > > if (!peraggstate->resulttypeByVal && !*isnull && > > > > > !MemoryContextContains(CurrentMemoryContext, > > > > > > > > > > DatumGetPointer(*result))) > > > > > seems somewhat problematic to me. MemoryContextContains() can give > > > > > false positives when used on memory that's not a distinctly allocated > > > > > chunk, and if so, we violate memory lifetime rules. It's quite > > > > > unlikely, given the required bit patterns, but nonetheless it's making > > > > > me somewhat uncomfortable. > > > > > > > > > > Do others think this isn't an issue and we can just live with it? > > > > > > > > I think it's 100% broken to call MemoryContextContains() on something > > > > that's not guaranteed to be a palloc'd chunk. > > > > > > I agree, but to me it seems the only fix would be to just yank out the > > > whole optimization? > > > > Dunno, haven't looked into it. > > > > I think it might be fixable by adding a flag into the chunk, with 'true' for > regular allocations, and 'false' for the optimized ones. And then only use > MemoryContextContains() for 'flag=true' chunks.
I'm not quite following here. We only get a Datum and the knowledge that it's a pass-by-ref argument, so we really don't know that much. We could create an "EmbeddedDatum" type that has a preceding chunk header (appropriately for the version), that just gets zeroed out at start. Is that what you mean? > The question however is whether this won't make the optimization pointless. > I also, wonder how much we save by this optimization and how widely it's > used? Can someone point me to some numbers? I don't recall any recent numbers. I'm more than a bit doubful that it really matters - it's only used for the results of aggregate/window functions, and surely they've a good chunk of their own overhead... Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
