Tom Lane <t...@sss.pgh.pa.us> wrote: > > Gavin Wahl wrote: > >> It seems trivial to accelerate a MAX or MIN query with a BRIN index. You > >> just find the page range with the largest/smallest value, and then only > >> scan that one. Would that be hard to implement? I'm interested in > working > >> on it if someone can give me some pointers. > > I think this proposal is fairly broken anyway. The page range with the > largest max-value may once have contained the largest live row, but > there's no guarantee that it still does. It might even be completely > empty. You could imagine an algorithm like this: > > 1. Find page-range with largest max. Scan it to identify live row with > largest value. If *no* live values, find page-range with next largest > max, repeat until no page ranges remain (whereupon return NULL). > > 2. For each remaining page-range whose indexed max exceeds the value > currently in hand, scan that page-range to see if any value exceeds > the one in hand, replacing the value if so. > > This'd probably allow you to omit scanning some of the page-ranges > in the table, but in a lot of cases you'd end up scanning many of them; > and you'd need a lot of working state to remember which ranges you'd > already looked at. It'd certainly always be a lot more expensive than > answering the same question with a btree index, because in no case do > you get to avoid scanning the entire contents of the index. > > regards, tom lane >
Thanks Tom, A b-tree index would certainly be faster for ordering. But in scenarios where you have huge datasets that can't afford the space or update time required for b-tree, could such a BRIN-accelerated ordering algorithm at least be faster than ordering with no index? Darren Lafreniere
