Monitoring and Detecting Lock Contention pdf

After completing this lesson, you should be able to do the following: • Define levels of locking • Identify causes of contention • Prevent locking problems • Use Oracle utilities to dete

Copyright © Oracle Corporation, 2002 All rights reserved.

Monitoring and Detecting

Lock Contention

After completing this lesson, you should be able to do the following:

• Define levels of locking

• Identify causes of contention

• Prevent locking problems

• Use Oracle utilities to detect lock contention

• Resolve contention in an emergency

• Resolve deadlock conditions

17-3 Copyright © Oracle Corporation, 2002 All rights reserved.

Locking Mechanism

• Automatic management

• High level of data concurrency

– Row-level locks for DML transactions – No locks required for queries

• Multi-version consistency

• Exclusive and Share lock modes

• Locks held until commit or rollback operations

are performed

Data Concurrency

Transaction 1

SQL> SELECT salary

2 FROM employees

3 WHERE id = 10;

SALARY 1000

SQL> UPDATE employees

2 SET salary=salary*1.1

3 WHERE id= 24878;

1 row updated.

SQL> UPDATE employees

2 SET salary=salary+1200;

13120 rows updated.

SQL> UPDATE employees

2 SET salary=salary*1.1

3 WHERE id= 24877;

1 row updated.

Transaction 2

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Two Types of Locks

• DML or data locks:

– Table-level locks – Row-level locks

• DDL or dictionary locks

(TM)

(TX)

DML Locks

A DML transaction gets at least two locks:

• A shared table lock

• An exclusive row lock

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Enqueue Mechanism

The enqueue mechanism keeps track of:

• Users waiting for locks

• The requested lock mode

• The order in which users requested the lock

Table Lock Modes

These table lock modes are automatically assigned

by the Oracle server:

• Row Exclusive (RX): INSERT, UPDATE, DELETE

• Row Share (RS): SELECT FOR UPDATE

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Manually Locking a Table

Manually acquired in LOCK TABLE statement:

• Share (S)

– No DML operations allowed – Implicitly used for referential integrity

SQL> LOCK TABLE hr.employees IN share MODE;

Manually Locking a Table

• Share Row Exclusive (SRX)

– No DML operations or Share mode allowed – Implicitly used for referential integrity

– No index is required on the foreign key column in

the child table

• Exclusive (X)

– No DML or DDL operations allowed by other sessions

– No manual locks allowed by other sessions – Queries are allowed

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DML Locks in Blocks

Row 6 Block Header

Lock bytes

Row 1

1

2

TX slot 1 TX slot 2

DDL Locks

• Exclusive DDL locks are required for:

– DROP TABLE statements

– ALTER TABLE statements

– (The lock is released when the DDL statement

completes.)

• Shared DDL locks are required for:

– CREATE PROCEDURE statements

– AUDIT statements

– (The lock is released when the DDL parse

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Possible Causes of Lock Contention

• Unnecessarily high locking levels

• Long-running transactions

• Uncommitted changes

• Other products imposing higher-level locks

Diagnostic Tools for Monitoring

Locking Activity

Transaction 1

UPDATE employees

SET salary =

salary x 1.1;

Transaction 2 Transaction 3

v$lock

v$locked_object

UPDATE employees SET salary =

salary x 1.1 WHERE empno = 1000; UPDATE employees

SET salary = salary x 1.1

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Transaction 1

Guidelines for Resolving Contention

UPDATE employees SET salary = salary x 1.1 WHERE empno = 1000;

> COMMIT/ROLLBACK ;

UPDATE employees SET salary = salary x 1.1 WHERE empno = 1000;

1 row updated;

Transaction 2

> ALTER SYSTEM KILL SESSION ‘10,23’;

9:00

9:05 10:30 11:30

Performance Manager: Locks

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Transaction

2

Transaction

1

Deadlocks

UPDATE employees

SET salary = salary x 1.1

WHERE empno = = 1000;

UPDATE employees

SET salary = salary x 1.1

WHERE empno = 2000;

ORA-00060:

Deadlock detected while

waiting for resource

UPDATE employees SET manager = 1342 WHERE empno = 2000; UPDATE employees SET manager = 1342 WHERE empno = 1000;

9:00

9:15

9:16

ORA-00060:

Deadlock detected while waiting for resource

Trace file

Server process

SID_ora_PID.trc

UNIX

17-26 Copyright © Oracle Corporation, 2002 All rights reserved.

Summary

In this lesson, you should have learned to do the following:

• Define levels of locking

• Identify causes of contention

• Prevent locking problems

• Use Oracle utilities to detect lock contention

• Resolve contention in an emergency

• Resolve deadlock conditions