When it is possible to push a selection to both arguments o

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When it is possible to push a selection to both arguments of a binary operator, we need to decide whether or not to do so. How would the existence of indexes on one of the arguments affect our choice? Consider, for instance, an expression sigma_C (R S), where there is an index on S.

Solution

If the index on S assists us find tuples that suit the condition C, and then certainly we should push the selection to S. Let the effect of the selection on S be a relation S\', which is a subset of S. Assuming there is no index on R that assists with condition C, then if we push the selection to R as well, we still have to intersect that result with S\'. though, if we do not push the selection to R, then all that leftovers is intersecting R with S\'. presenting also the selection or the intersection on R needs that we read all of R, so we are improved off performing currently the intersection quite than the selection advantage a intersection