Hi Julian, You are right it should produce zero rows not NULL. Thanks for the correction.
Vineet On Wed, Jun 26, 2019 at 4:49 PM Julian Hyde <[email protected]> wrote: > > Select count(*) from empty_table group by <constant> will produce NULL > > Really? I thought it should produce zero rows. > > Hsqldb: > > > select count(*) from "foodmart"."days" where false group by true; > +-----------------+ > | C1 | > +-----------------+ > +-----------------+ > No rows selected (0.001 seconds) > > > Julian > > > > On Jun 26, 2019, at 1:12 PM, Vineet Garg <[email protected]> wrote: > > > > Hello Krzysztof, > > > > The rewrite you mention in Hive was done in HIVE-19674 > > <https://issues.apache.org/jira/browse/HIVE-19674> to be able to push > such > > group by to Druid. Currently there is no way to disable this rewrite. > > > > As for removing Group by <constant>, there are rules/rewrites which can > > reduce grouping keys by removing constants but removing whole group by is > > not safe since it can lead to semantically different query. > > e.g. Select count(*) from empty_table group by <constant> will produce > NULL > > but Select count(*) from empty_table will produce 0. > > > > P.S. There was a bug in HIVE-19674' patch which was further fixed by > > HIVE-21539 <https://issues.apache.org/jira/browse/HIVE-21539>. > > > > Regards, > > Vineet Garg > > > > On Wed, Jun 26, 2019 at 7:08 AM Haisheng Yuan <[email protected]> > > wrote: > > > >> Calcite has the rule that does the work. But you can't remove the group > by > >> clause if the constant is the only group key. The semantic is different > >> without group key. Try it on empty relation, you will see the > difference. > >> > >> > >> > >> > >> > >> Thanks~ > >> Haisheng > >> Yuan------------------------------------------------------------------ > >> 发件人：Krzysztof Zarzycki<[email protected]> > >> 日 期：2019年06月26日 21:52:41 > >> 收件人：<[email protected]> > >> 主 题：Modify Calcite Planner in Hive to remove GROUP BY <constant> > >> > >> Hello, > >> > >> While the question I have might look like regards to Hive, I believe is > >> more about Calcite. I need to add a Calcite plan rule to Hive, that > removes > >> "Group by" clause when it groups by some constant value (GROUP BY TRUE > more > >> precisely). As far as I believe, the query semantically is the same. > >> Could anyone on this mailing list help me how to do it properly? While > I'm > >> an experienced java engineer, I have no clue how to achieve this. > >> I was trying to modify hive code to do this myself, but unfortunately I > got > >> only NullPointerExceptions. > >> > >> > >> More context below: > >> I want to use JdbcStorageHandler in Hive, that connects to Apache Kylin > and > >> forward queries there. Then I put Tableau on top of Hive. Unfortunately, > >> the queries produced by Tableau to Hive and then reproduced by Calcite > >> Planner to Kylin, cannot be handled by Kylin (which BTW uses Calcite as > >> well). I disabled some of the hive optimizations which fixed some of my > >> queries. But I'm stuck on one I cannot disable. Tableau generates a > query > >> with "GROUP BY 1.000000...01" , that is translated to "GROUP BY TRUE", > by > >> Hive/Calcite. But neither of those can be handled by Kylin. I got an > idea > >> that I will remove GROUP BY completely, because in my understanding it's > >> unecessary. > >> > >> I will be very grateful for your help, > >> Kind Regards, > >> Krzysztof > >> > >> > >
