Oops, my bad. I just noticed you did indicate that the query plan shows usage of a hash agg.
-- Zelaine On Wed, May 25, 2016 at 7:48 PM, Zelaine Fong <zf...@maprtech.com> wrote: > What does the explain plan show? I.e., is the group by being done via a > hash agg or a streaming agg? If it's a streaming agg, then you still have > to sort the entire data set before you reduce it down to a single group. > That would explain the increase in memory as you add group by keys. > > -- Zelaine > > On Wed, May 25, 2016 at 5:50 PM, rahul challapalli < > challapallira...@gmail.com> wrote: > >> I am trying to understand the memory usage patterns for hash aggregate. >> The >> below query completes in 9.163 seconds and uses 24 MB of memory for >> hash-aggregate (according to profile) >> >> select max(d.l_linenumber) from (select l_linenumber, 'asdf' c1, 'kfjhl' >> c2, 'reyui' c3, 'khdfs' c4, 'vkhj' c5 from mem_heavy1) d group by d.c1, >> d.c2, d.c3, d.c4, d.c5; >> >> Adding one more constant column to the group by, the below query takes >> 11.638 seconds and uses 29 MB of ram >> >> select max(d.l_linenumber) from (select l_linenumber, 'asdf' c1, 'kfjhl' >> c2, 'reyui' c3, 'khdfs' c4, 'vkhj' c5, 'bmkr' c6 from mem_heavy1) d group >> by d.c1, d.c2, d.c3, d.c4, d.c5, d.c6; >> >> The below query with one more constant column added to group by 14.622 >> seconds and uses 33 MB memory >> >> select max(d.l_linenumber) from (select l_linenumber, 'asdf' c1, 'kfjhl' >> c2, 'reyui' c3, 'khdfs' c4, 'vkhj' c5, 'bmkr' c6, 'ciuh' c7 from >> mem_heavy1) d group by d.c1, d.c2, d.c3, d.c4, d.c5, d.c6, d.c7; >> >> >> As you can see, there is only one disctinct group in all the above cases. >> It looks like the memory usage is proportional to no of elements in the >> group by clause. Is this expected? >> >> Is the increase in time expected between the above queries? (As we did not >> introduce any new groups) >> >> - Rahul >> > >
