Task $7$

We are doing inner join for three relations $R1(A,B),R2(B,C),$ and R$3($C$,$D$).$

And we know the following:

For R$1$: $T(R1)=2000,V(R1,A)=50,V(R1,B)=100$

For R$2$: $T(R2)=3000,V(R2,B)=200,V(R2,C)=300$

For R$3$: $T(R3)=1000,V(R3,C)=90,V(R3,D)=500$

Which two relations should we join first to optimize the query execution speed?