[GitHub] [druid] himanshug commented on issue #9122: Add SQL GROUPING SETS support.

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himanshug commented on issue #9122: Add SQL GROUPING SETS support.
URL: https://github.com/apache/druid/pull/9122#issuecomment-589828549
 
 
   > Alter behavior so limitSpec is applied after subtotalsSpec, rather than applied to
   each grouping set. This is more in line with SQL standard behavior. I think it is okay
   to make this change, since the old behavior was not documented, so users should
   hopefully not be depending on it.
   
   saw this today, I think, with original behavior, I tried to replicate semantics equivalent of sending separate  queries for each subtotalsSpec individual (pre subtotalsSpec world) . That was the usecase at hand when subtotalsSpec feature  was created. However, I can't recall if that usecase 
    did have needs for limitSpec.  @a2l007 do you know if there is dependency of  limit/sort spec  behavior when subtotalsSpecs  are used  ?
   If a limitSpec (containing sort  spec) is applied in  the end then  that will mixup the rows and it would be difficult to identify  which rows  came from overall or which subtotalSpec .  Oracle appears to support  GROUPING function (Ref: https://docs.oracle.com/cd/B28359_01/server.111/b28314/tdpdw_sql.htm#TDPDW00714 ) to solve that problem in addition to having the behavior you described from `ROLLUP` and `CUBE` functions by adding '1' for dims not present in subtotal spec for rows created by  subtotalSpec.
   For native query users, what would be the alternative to achieve similar effect? For  this reason,  it might be good to have a feature flag for enabling `old` behavior .
   

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