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Query Cost for MA.Hashing

Multi-attribute hashing handles a range of query types, e.g. 

select * from R where a=1
select * from R where d=2
select * from R where b=3 and c=4
select * from R where a=5 and b=6 and c=7

A relation with n attributes has 2n different query types.

Different query types have different costs   (different no. of *'s)

Query distribution gives probability pQ of asking each query type Q.