On Sun, Apr 5, 2020 at 9:39 PM Dmitry Dolgov <9erthali...@gmail.com> wrote: > > > On Sun, Apr 05, 2020 at 04:30:51PM +0530, Dilip Kumar wrote: > > > > I was just wondering how the distinct will work with the "skip scan" > > if we have some filter? I mean every time we select the unique row > > based on the prefix key and that might get rejected by an external > > filter right? > > Not exactly. In the case of index-only scan, we skipping to the first > unique position, and then use already existing functionality > (_bt_readpage with stepping to the next pages) to filter out those > records that do not pass the condition.
I agree but that will work if we have a valid index clause, but "b%100=0" condition will not create an index clause, right? However, if we change the query to select distinct(a) from t where b=100 then it works fine because this condition will create an index clause. There are even couple of tests > in the patch for this. In case of index scan, when there are some > conditions, current implementation do not consider skipping. > > > So I tried an example to check this. > > Can you tell on which version of the patch you were testing? I have tested on v33. -- Regards, Dilip Kumar EnterpriseDB: http://www.enterprisedb.com
