CMPS 181, Database Systems II, Final Exam, Spring 2016 Instructor: Shel Finkelstein. Student ID: UCSC

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1 CMPS 181, Database Systems II, Final Exam, Spring 2016 Instructor: Shel Finkelstein Student Name: Student ID: UCSC Final Points: Part Max Points Points I 15 II 29 III 31 IV 19 V 16 Total 110 Closed book, but okay to bring a single two-sided 8.5 x 11 sheet of paper with as much info written on it as you can fit and read unassisted. You may write answers on the backs of previous pages of the exam if you need extra space. Please indicate which questions you re answering if you do this.

2 Part I: Storage and Records Question 1 (6 points): a) What does it mean to say that a page is pinned in the database buffer pool? b) How does a page become pinned? c) How does a page become unpinned? Answers 1a, b and c:

3 Question 2 (3 points): True or False: Although we used (pageid, slotid) as the rid for records, a better approach for variable-length records would be to use (pageid, offset), where offset is the record s position on the page. Answer 2: Question 3 (6 points): We talked about two general approaches to faulttolerant RAID, mirroring and parity. a) Explain what parity is, and how it enables recovery from a failure. b) What s an advantage that parity has versus mirroring? Answer 3:

4 Part II: Access Paths and Query Optimization #1 Question 4 (3 points): True or False: Even though finding a good way to execute a query is important, database optimizers don t always estimate the cost of every query plan, because there are too many plans. Answer 4: Question 5 (8 points): Two of the join algorithms that we discussed were (indexed) nested loop join and merge join. a) Give an example of two relations, a join query, indexes and cardinalities where a plan using nested loop join has lower cost than any merge join plan. Give cost estimates to demonstrate the lower cost. b) Give another (could be completely different) example of two relations, a join query, indexes and cardinalities, where a merge join plan has lower cost than any (indexed) nested loop join. Give cost estimates to demonstrate the lower cost. Answers 5a and b:

5 Question 6 (4 points): Pipelining is one way of introducing parallelism into query execution, allowing partial results from an early operation to be processed by a later operation. But sometimes, pipelines can be blocked, meaning that the early operation must be fully completed before the later operation can be started. a) Describe an example of pipelining that allows parallelism. b) Describe an example where a pipeline is blocked. Answers 6a and b:

6 Question 7 (3 points): The System R optimizer avoids doing Cartesian products whenever possible. Explain why this is a good approach. Answer 7:

7 Question 8 (6 points): Hash Join has two stages, a) the Build Stage and b) the Match Stage. Give a brief description (using pictures to help, if you d like) of what happens in each of the two stages a) and b). Be sure to explain why the hash function used in Match is different than the one that was used in Build. Answer 8a and b:

8 Question 9 (5 points): The Sailors relation has 100,000 tuples in it. The rating column for Sailors has low value 1 and high value 10, and age column for Sailors has low value 21 and high value 60. Both rating and age are integers. Consider the query: SELECT * FROM Sailors WHERE rating = 7 AND age <= 30; a) How many tuples would a System R style optimizer assume are in the result of that query? b) Suppose that there are 50,000 Sailors whose age is 30 or less. Explain specifically how histograms could help improve the optimizer s size estimate for this query, and give the size estimate when using histograms. Answers 9a and b:

9 Part III: Transaction Management Question 10 (3 points): True or False: When a transaction executes with Repeatable Read as the Isolation Level, then all data that the transaction reads existed simultaneously at a single logical point in time. (Data at a logical point in time means the database state after some fixed set of transactions had committed, with no uncommitted data.) Answer 10: Question 11: (6 points): a) What are Cascading Rollbacks? b) Give an example showing how a Cascading Rollback could arise in a database system that didn t prevent Cascading Rollbacks. c) What do database systems do to avoid Cascading Rollbacks? Give an explanation, not just a term or phrase. Answers 11a, b and c:

10 Question 12 (6 points): a) What is a deadlock? b) Suppose that your database system assigned priorities to transactions when they began executing, and it handled deadlocks by rolling back the transaction T involved in the deadlock that had the lowest priority (p). That Transaction T would be restarted after a small interval, with the same priority p. What would be a major disadvantage of this approach? c) Suppose that your database system handled deadlocks by rolling back the transaction involved in the deadlock that had started at the earliest time. What would be a major disadvantage of this approach? Answers 12 a, b and c:

11 Question 13 (3 points): True or False: In the locking protocols that we studied, shared and exclusive locks on B-trees are handled using two-phase locking. Answer 13: Question 14 (4 points): a) Describe a significant advantage of locking over optimistic concurrency control. b) Describe a significant advantage of optimistic concurrency control over locking. Answers 14a and b:

12 Question 15 (6 points): a) When does Write-Ahead Logging force the database log to disk? Assume that this is a transaction on a single database, not a distributed transaction. b) Why does the log need to be forced then? (Be clear about what could go wrong if the log wasn t forced.) c) Database systems enforce ordering requirements on the records in the log, and on forces of the log. Describe those requirements. Answers 15a, b and c:

13 Question 16 (3 points): Distributed commit protocols are complex. For a distributed transaction T updating 3 databases (DB1, DB2 and DB3), it would be much simpler just to commit T s updates on DB1, and if that succeeded, commit T s updates on DB2, and if that also succeeded, commit T s updates on DB3. Explain significant disadvantages of this much simpler approach. Answer 16:

14 Part IV: Access Paths and Query Optimization #2 Question 17 (3 points): True or False: When you search an R-tree and reach an entry in a leaf node, the entry that you found might not satisfy the original query, so you have to do some additional checking. Answer 17: Question 18 (4 points): We focused on queries with AND predicates, but indexes can be used for queries with OR as well. Consider the relation: Employees(empnum:INTEGER, name:char(20), salary:integer, title:char(6)) Suppose that there is a B-tree index on salary and a hash index on title. Explain how a database system could efficiently use these two access paths together to return the correct answer to the following query: SELECT empnum, name FROM Employees WHERE salary < 9000 OR title = INTERN ; Answer 18:

15 Question 19 (12 points): The Executives relation has the attributes execname, title, deptname, and address; execname is the key. All attributes are string fields of the same length, 100 bytes, so that records are 400 bytes. There are 10 buffer pages available. The relation contains 10,000 pages. Pages are size 4K and you can assume that all pages are full. (So how many records are there per page?) Also, assume that the leaf pages of a B-tree index on one-column would have 2500 pages. Suppose that a database systems has to execute the following query: SELECT DISTINCT deptname, FROM Executives; Answer the following questions. If you need to make some addition assumptions to answer these questions, say what they are. Some of these questions are tough, and you ll be graded in part based on how sensible your answers (and assumptions are). 19a) Give a rough estimate for the cost of executing the query by sorting on deptname. Explain your calculation. Note that in the initial sorting pass, you don t have to keep all the attributes, only deptname. Answer 19a:

16 19b) Suppose that there is a clustered index on deptname. Give an estimate of the cost of answering the query using that index. Explain your calculation. Answer 19b):

17 19c) Suppose that you had a hash index on deptname. Describe a way to execute the query using that hash index, and estimate its cost. Remember that you can state additional reasonable assumptions if you want to do so. Explain your calculation. Answer 19c):

18 Part V: Parallelization and Distribution Question 20 (6 points): a) What are Shared-Everything and Shared-Nothing database architectures? b) What is an advantage of Shared-Everything database architecture over Shared-Nothing database architecture? c) What is an advantage of Shared-Nothing database architecture over Shared-Everything database architecture? Answers 20a, b and c:

19 Question 21 (4 points): Partitions separate relations into pieces, sometimes called shards. a) What is partition pruning? b) Give an example of a query and a partitioned relation where you can use partition pruning to make processing cheaper. Answers 21a and b:

20 Question 22 (6 points): This question is about Two-Phase Commit across multiple database sites. a) After a subordinate database site replies to a PREPARE saying Yes, what are its obligations? b) What did it have to do before it replied Yes? c) After the coordinator database site receives YES votes from all subordinate sites and writes a COMMIT record to its log, does it have any obligations to the subordinate sites? If so, what are they? Answer 22a, b and c: