Stamatis, Just FYI, maybe it will be useful for you, Drill uses *noDAG: true *as default value for HepPlanner [1]. After changing it to false, a lot of Drill unit tests failed [2].
[1] https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/planner/sql/handlers/DefaultSqlHandler.java#L416 [2] https://travis-ci.org/vdiravka/drill/jobs/494499462 Kind regards Vitalii On Fri, Feb 15, 2019 at 2:43 PM Stamatis Zampetakis <zabe...@gmail.com> wrote: > FYI, what I concluded by going through the code and the various test cases > is the following. > > By allowing DAGs the planner can detect common sub expressions in queries > and re-use an existing result without re-applying a rule if that is not > necessary. This should lead to fewer object creations and rule > applications, which may in turn lead to improved performance. In the > existing use cases noDag=false should appear more often since it is the > default value for two out of three constructors in the HepPlanner. > > In principle it seems that using or not using DAGs should give the same > expression in the end so I would say that using DAGs is always a better > option. I tried setting noDag to be always true but various test fail with > StackOverflowError so it seems there are rules who tend to execute infinite > number of times as a result of this change. I would tend to thing that this > is a bug but I didn't look further. > > Στις Τρί, 12 Φεβ 2019 στις 9:35 μ.μ., ο/η Julian Hyde <jh...@apache.org> > έγραψε: > > > I don’t recall. > > > > Could you review the tests and see whether tests tend to use noDag=true > or > > false most of the time? Are there any tests that use the less popular > > value, and if so, is there a particular reason that those tests use that > > option? > > > > Julian > > > > > > > On Feb 12, 2019, at 6:47 AM, Stamatis Zampetakis <zabe...@gmail.com> > > wrote: > > > > > > Hi all, > > > > > > I don't understand what is the correct way to set the noDag [1] > parameter > > > in HepPlanner. I understand what it does (internal query graph becomes > a > > > tree or a DAG) but I don't see why should I use the one or the other > and > > > when. > > > > > > Is it performance related? > > > Are there implications on the rules that can be used with the planner? > > > Does it limit the class of queries that need to be transformed? > > > > > > Thanks in advance, > > > Stamatis > > > > > > [1] > > > > > > https://github.com/apache/calcite/blob/883666929478aabe07ee5b9e572c43a6f1a703e2/core/src/main/java/org/apache/calcite/plan/hep/HepPlanner.java#L131 > > > > >
