On Fri, Jul 8, 2016 at 9:49 AM, Robert Haas <robertmh...@gmail.com> wrote: > On Thu, Jul 7, 2016 at 4:56 PM, Bill Moran <wmo...@potentialtech.com> wrote: >> SELECT DISTINCT size FROM grue; >> >> Always does a seq scan on Postgres 9.5.2. (Yes, I know we're >> a patch behind, the upgrade is on the schedule) on >> Ubuntu 14. >> >> I would expect it to be possible, and significantly more >> efficient to do an index scan for that query. > > [...] > > We're probably missing a few tricks on queries of this type. If the > index-traversal machinery had a mechanism to skip quickly to the next > distinct value, that could be used here: walk up the btree until you > find a page that contains keyspace not equal to the current key, then > walk back down until you find the first leaf page that contains such a > value. That would potentially let you step over large chunks of the > index without actually examining all the leaf pages, which for a query > like this seems like it could be a big win.
FWIW I messed around with prototyping this idea here: https://www.postgresql.org/message-id/cadlwmxwalk8npzqdnrqipnrzanic7nxykynrkzo_vxyr8en...@mail.gmail.com I hope to return to that and some related ideas eventually as I learn more about the relevant areas of the source code, if someone doesn't beat me to it. https://wiki.postgresql.org/wiki/Loose_indexscan shows a recursive CTE that does the same thing at a higher level. -- Thomas Munro http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers