Quiz: Query Optimization _ Consider the relations r1( A, B, C), r2(C, D, E ), and r3( E , F), with p

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Quiz: Query Optimization Your name:

1.

Consider the relations r1( A, B, C), r2(C, D, E ), and r3( E , F), with primary keys A, C, and E, respectively. Assume that r1 has 1000 tuples, r2 has 1500 tuples, and r3 has 750 tuples. Estimate the size of r1 ⋈ r2 ⋈ r3, and give an efficient plan for computing the join.

Answer: -

No matter which way we join them, the joining results will be the same which will have at most 1000 tuples. (referring to slide 48/Lecture10 in Week 9) (r1 ⋈ r2) ⋈ r3

2. Consider the following relational schema and SQL query. The schema captures information about employees, departments, and company finances (organized on a per department basis).

Emp(eid: integer, did:integer, sal: integer, hobby:char(20)) Dept(did:integer, dname:char(20), floor:integer, phone:char(10)) Finance(did:integer, budget:real, sales: real, expenses:real) Consider the following query: SELECT D.dname, F.budget FROM Emp E, Dept D, Finance F WHERE E.did = D.did AND D.did = F.did AND D.floor = 1 AND E.sal ≥ 59000 AND E.hobby = ‘yodeling’ a. Identify a relational algebra tree (or a relational algebra expression if you prefer) that reflects the order of operations a decent query optimizer would choose. b. List the join orders (i.e., orders in which pairs of relations can be joined to compute the query result) that a relational query optimizer will consider. (Assume that the optimizer follows the heuristic of never considering plans that require the computation of cross-products.) Briefly explain how you arrived at your list.

Answer: a.

b. There are 2 join orders considered, assuming that the optimizer only consider left-deep joins and ignores cross-products: (D,E,F) and (D,F,E) 1 This study source was downloaded by 100000850872992 from CourseHero.com on 04-03-2023 02:13:40 GMT -05:00

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3. Consider the bank database in the following figure, where the primary keys are underlined.

Construct the following SQL queries for this relational database. a. Write a nested query on the relation account to find, for each branch with name starting with ‘B’, all accounts with the maximum balance at the branch.

b. Rewrite the preceding query, without using a nested subquery; in order words, decorrelate the query. c. Evaluate your two queries, which one is more efficient?

Answer: a. The nested query is as follows: select S.acount_number from

account S

where S.branch_name like ’B%’ and S.balance = (select max(T.balance) from account T where T.branch_name = S.branch_name) b. The query is as follows: create table t1 as select branch_name, max(balance) from account group by branch_name select account_number from account, t1 where account.branch_name like ’B%’ and

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account.branch_name = t1.branch_name and account.balance = t1.balance

c. query b.

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