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 >
