> 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 <vg...@apache.org> 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 <h.y...@alibaba-inc.com> > 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<k.zarzy...@gmail.com> >> 日 期：2019年06月26日 21:52:41 >> 收件人：<dev@calcite.apache.org> >> 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 >> >>
