Its using hash aggregation. On May 25, 2016 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 > > >
