On Thu, Mar 27, 2014 at 10:00 PM, Tomas Vondra <t...@fuzzy.cz> wrote: > The patch also does one more thing - it changes how the arrays (in the > aggregate state) grow. Originally it worked like this > > /* initial size */ > astate->alen = 64; > > /* when full, grow exponentially */ > if (astate->nelems >= astate->alen) > astate->alen *= 2; > > so the array length would grow like this 64 -> 128 -> 256 -> 512 ... > (note we're talking about elements, not bytes, so with with 32-bit > integers it's actually 256B -> 512B -> 1024B -> ...). > > While I do understand the point of this (minimizing palloc overhead), I > find this pretty dangerous, especially in case of array_agg(). I've > modified the growth like this: > > /* initial size */ > astate->alen = 4; > > /* when full, grow exponentially */ > if (astate->nelems >= astate->alen) > astate->alen += 4; > > I admit that might be a bit too aggressive, and maybe there's a better > way to do this - with better balance between safety and speed. I was > thinking about something like this: > > > /* initial size */ > astate->alen = 4; > > /* when full, grow exponentially */ > if (astate->nelems >= astate->alen) > if (astate->alen < 128) > astate->alen *= 2; > else > astate->alen += 128; > > i.e. initial size with exponential growth, but capped at 128B.
So I think this kind of thing is very sensible, but the last time I suggested something similar, I got told "no": http://www.postgresql.org/message-id/caeylb_wlght7yjlare9ppert5rkd5zjbb15te+kpgejgqko...@mail.gmail.com But I think you're right and the objections previously raised are wrong. I suspect that the point at which we should stop doubling is higher than 128 elements, because that's only 8kB, which really isn't that big - and the idea that the resizing overhead takes only amortized constant time is surely appealing. But I still think that doubling *forever* is a bad idea, here and there. The fact that we've written the code that way in lots of places doesn't make it the right algorithm. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
