Query Rewrites: Predicate Pushdown (through grouping)

Select bid, Max(age)

From Reserves R, Sailors S

Where R.sid=S.sid

GroupBy bid

Having Max(age) > 40

Select bid, Max(age)

From Reserves R, Sailors S

Where R.sid=S.sid and

S.age > 40

GroupBy bid

Having Max(age) > 40

For each boat, find the maximal age of sailors who’ve reserved it.

Advantage: the size of the join will be smaller.

Requires transformation rules specific to the grouping/aggregation

operators.

Won’t work if we replace Max by Min.

Previous slide Next slide Back to first slide View graphic version