$(30$ pts$)$ Consider the following database schema:

Sailors$($sid$,$ sname, rating, age$)$ Boats$($bib$,$ bname, color$)$ Reserves$($sid$,$ bid, day$)$

Reserves: $100,000$ tuples stored in $1,000$ pages. There

are $100$ different boats $($bid$\text{'}$s$).$ They are equally

popular li$.$e$.,$ each has the same number of

reservations$)$

Sailors: $1000$ tuples stored in $100$ pages. There are $10$

rating levels, each with the same number of sailors.

$12$ memory pages are available for the computation of the

execution plan in the right.

a$)$ Explain the computation of the block nested loop

join.

b$)$ Estimate the total I$/$O cost of this execution plan.