avantgardnerio commented on PR #5362:
URL: 
https://github.com/apache/arrow-datafusion/pull/5362#issuecomment-1440976811

   > often Hash Joins are used
   
   I checked, this is true for query 9. I was trying to find a broadly 
applicable example, but I'm having a difficult time.
   
   > all the predicates it can
   
   Unfortunately, it pushes them down after it splits the conjunctions. The 
indexes may not be helpful for partial predicates. By exposing this information 
to the planner, it can try pushing down ones that match the available indexes. 
As a side benefit, it might also help to decide when to use a sort-merge-join 
(without the sort) because PK & FK sides are already ordered correctly for the 
join.
   
   > non trivial to implement / test / get right.
   
   I was hoping this could be the first step towards that - giving existing 
index info to the planner certainly couldn't hurt?


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