[TRAFODION-2418] Allow group by push-down to a fact table

[zellerh]

Trafodion has a rule that pushes a groupby down over a join when possible, but it will not push the groupby to the left child of a join - it relies on join commutativity. This might have been ok 20 years ago when this was coded (by me...), but it isn't good for some situations today. Example:

select d.y, count(f.a), sum(f.b)
from big_fact f join small_dim d on f.x=d.y
where d.a = 1
group by d.y

The plan we would like is a hash join with the group by on big_fact as the left child. To do this, we need to remove the heuristic that prevents this form of push-down.

[Traf-Jenkins]

Check Test Started: https://jenkins.esgyn.com/job/Check-PR-master/1493/

[Traf-Jenkins]

Test Passed. https://jenkins.esgyn.com/job/Check-PR-master/1493/

[moscowgentalman]

+1
Looks good to me.

[DaveBirdsall]

+1 Looks good to me. Do you know what heuristic is preventing push down via the commutativity rule?

[zellerh]

No, I did not investigate in detail why we don't get the transformation with the current rules. It could be the multi-join rule or some other heuristic. To get the plan I wanted it would have had to apply the commutativity rule twice, once to enable the groupby pushdown and a second time to bring the fact table back to the left side.