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Grace Hash Join (cont)

Cost of grace hash join:
  • #pages in all partition files of Rel ≅ bRel   (maybe slightly more)
  • partition relation R ...   Cost  =  bR.Tr + bR.Tw  =  2bR
  • partition relation S ...   Cost  =  bS.Tr + bS.Tw  =  2bS
  • probe/join requires one scan of each (partitioned) relation
    Cost  =  bR + bS
  • all hashing and comparison occurs in memory   ⇒   ≅0 cost
Total Cost   =   2bR + 2bS + bR + bS   =   3 (bR + bS)