Deadlocks For $1000: Let’s Play Jeopardy!

Iggy FernandezDatabase Specialists, Inc.www.dbspecialists.com

TCOUG Fall Meeting 2008

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Speaker Qualifications

Oracle DBA at Database Specialists Editor of the Journal of the Northern California Oracle

Users Group Author of Beginning Oracle Database 11g

Administration

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Deadlock example—Overlapping data

Transaction A: Delete from child where child_ID=1;

Transaction B: Delete from child where child_ID=2;

Transaction A: Delete from child where child_ID=2;

Transaction A: Delete from child where child_ID=1;

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Deadlock example—non-overlapping data!

Transaction A: Delete from child where child_ID=1;

Transaction B: Delete from child where child_ID=2;

Transaction A: Delete from parent where parent_ID=1;

Transaction B: Delete from parent where parent_ID=2;

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Further Reading

Understanding and Interpreting Deadlocks or What to Do When You Encounter ORA-00060 by Mark Bobak. Available for download at the Oak Table website www.oaktable.net/userFiles.jsp

Metalink document 164661.1: ORA-60/Deadlocks Most Common Causes

Metalink document 62365.1: What to do with “ORA-60 Deadlock Detected” Errors.

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Deadlocks for 100

Which of the following situations is an example of an Oracle deadlock? Pick all that apply.

1. A situation in which a blocking session becomes inactive but does not release its locks.

2. A situation in which a blocking session is itself blocked by another session.

3. A situation in which two sessions are trying to modify a row of data that has previously been modified by the other.

4. A cycle in the “wait-for” diagram.5. A situation in which a session is blocking itself, i.e.,

“self-deadlock.”

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Deadlocks for 200

What kind of situation cannot result in an Oracle deadlock? Pick all that apply.

1. A situation involving a single table, because deadlocks cannot occur if everybody is modifying the same table.

2. A situation involving a single user, because sessions owned by the same user cannot deadlock with each other.

3. A situation involving a single session, because a session cannot deadlock with itself.

4. A situation not involving any locking activity whatsoever.

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Deadlocks for 300

Who receives the “ORA-00060: deadlock detected while waiting for resource” error message?

1. The participant that is blocking the others.2. Only one of the participants in the deadlock,

selected by Oracle.3. All the participants in the deadlock.

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Deadlocks for 400

What is the effect on any session that receives the “ORA-00060: deadlock detected while waiting for resource” error message?

1. The statement that it was executing is rolled back.

2. Its transaction is rolled back.3. It is terminated.4. It depends on the circumstances.

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Deadlocks for 500 (Daily Double)

What action must the database administrator take to resolve the deadlock?

1. Terminate the session that is blocking the others.

2. Terminate any one of the sessions participating in the deadlock.

3. Terminate all the sessions participating in the deadlock.

4. None of the above.

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Deadlocks for 600

What is the recommended action that a session should take when it receives an ORA-00060 error message?

1. Issue the ROLLBACK TO SAVEPOINT command and repeat the statement that it was executing when it received the ORA-00060 error message.

2. Repeat its entire transaction; there is no need to issue an explicit ROLLBACK command because Oracle has already rolled back the entire transaction.

3. Repeat the entire transaction; the session must first issue an explicit ROLLBACK command because Oracle has not rolled back the transaction.

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Deadlocks for 700

What causes a deadlock in Oracle? Pick all that apply.

1. Contention for the same rows of data by multiple sessions.

2. Physical database design issues such as unindexed foreign keys.

3. Architectural issues such as block-level locking.

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Deadlocks for 800

How does Oracle detect that a deadlock has occurred?

1. It checks for deadlocks whenever a session cannot acquire a lock.

2. It checks for deadlocks at regular intervals.3. It checks for deadlocks whenever the wait

time of a session has exceeded a certain threshold.

4. It checks for deadlocks whenever the number of waiting sessions has exceeded a certain threshold.

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Deadlocks for 900

How can you obtain a list of all the rows locked by a transaction? Pick all that apply.

1. By querying V$LOCK in order to identify the locked rows.

2. By querying V$LOCKED_OBJECT in order to identify the locked rows.

3. By querying DBA_LOCK in order to identify the locked rows.

4. None of the above.

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Deadlocks for 1000!

A transaction issues a SAVEPOINT command and then locks a data row that is subsequently required by another transaction. What happens if the blocking transaction issues a ROLLBACK TO SAVEPOINT command?

1. The blocking transaction reverses the changes made to the data row but does not release its lock on the row.

2. The blocking transaction reverses the changes made to the data row and releases its lock on the row; the blocked transaction is automatically unblocked.

3. The blocking transaction reverses the changes made to the data row and releases its lock on the row, but the blocked transaction remains blocked.

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White Paper

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Contact Information

Iggy FernandezDatabase Specialists, Inc.388 Market Street, Suite 400San Francisco, CA 94111

Tel: 415-344-0500 Ext. 43Email: ifernandez@dbspecialists.comWeb: www.dbspecialists.com

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minimize risk, and reduce costs

Database Specialists helps you increase uptime, improve performance,

minimize risk, and reduce costs

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About Database Specialists

Database Specialists, Inc. provides Oracle database consulting in Solaris, Linux, HP-UX, AIX, and Windows environments.

Our DBA Pro offering and Database Rx™ tools provide remote database support and 24/7 coverage at an attractive price point.

We specialize in short term projects including upgrades, performance tuning and health checks. Our Oracle DBAs each have a minimum of 10 years of Oracle experience with a focus on

Oracle technology, mission-critical production support and RAC environments. Database Specialists is US-based.

Database Specialists helps you increase uptime, improve performance,

minimize risk, and reduce costs

Database Specialists helps you increase uptime, improve performance,

minimize risk, and reduce costs

Deadlocks For $1000: Let’s Play Jeopardy!

Iggy FernandezDatabase Specialists, Inc.www.dbspecialists.com

TCOUG Fall Meeting 2008