Re: [I] Push down entire hash table from HashJoinExec into scans [datafusion]

["rkrishn7 (via GitHub)" <gi...@apache.org>]

rkrishn7 commented on issue #17171:
URL: https://github.com/apache/datafusion/issues/17171#issuecomment-3257197579

   Hello! This is quite interesting.
   
   > Should we always push down both the hash table and bounds?
   
   > Should we push down the entire hash table as is or should we build bloom filters, an IN LIST expression or other?
   
   My initial thought is that the number of distinct values (NDV) on the build side could serve as a useful signal for which filter to push down. For example:
   
   - If the join column has high cardinality, the values are likely fairly random, in which case pushing down an `IN LIST` may be more effective. I _think_ existing pruning machinery already handles `IN LIST` quite well — e.g. enabling Parquet row group pruning via Bloom filters.
   
   - On the other hand, if the column shows low variance, then min/max statistics may provide better pruning.
   
   Curious to hear your thoughts on this approach!


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