By the time the query is translated into relational algebra, SELECT DISTINCT will have been converted into an Aggregate with no aggregate functions.
So, for instance, select distinct deptno from emp and select deptno from emp group by deptno will be indistinguishable if you look at the relational algebra. You should write a rule that matches a Sort on top of an Aggregate that uses the same keys. Also, I don't know whether your system supports primary keys. But if, for example, empno is unique, then you would want to be able to evaluate select empno from emp order by empno even though there is no explicit SELECT DISTINCT. You can use the RelMdUniqueKeys metadata to figure out whether the sort key is already unique. Julian On Fri, May 13, 2016 at 8:22 AM, Tzolov, Christian <ctzo...@pivotal.io> wrote: > Hi there, > > I am working on a Calcite adapter for a datastore that supports a limited > subset of SQL expressions. My goal is to push down the supported expression. > > This datastore supports "ORDER BY” only if the SELECT DISTINCT expression is > used. > > What would be the best way to check if the select DISTINCT keyword is used > from within my SortRule implementation? > > Thanks, > Christian >
