less09 data - managing data and concurrency

7/30/2019 Less09 Data - Managing Data and Concurrency

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Copyright 2008, Oracle. All rights reserved.

Managing Data and Concurrency


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Copyright 2008, Oracle. All rights reserved.9 - 2

Objectives

After completing this lesson, you should be able to: Manage data by using SQL Identify and administer PL/SQL objects

Describe triggers and triggering events Monitor and resolve locking conflicts


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Manipulating Data by Using SQL .

SQL> INSERT INTO employees VALUES 2 (9999,'Bob','Builder','[email protected]',NULL,SYSDATE,

3 'IT_PROG',NULL,NULL,100,90);1 row created.

SQL> UPDATE employees SET SALARY=6000 2 WHERE EMPLOYEE_ID = 9999;

1 row updated.

SQL> DELETE from employees2 WHERE EMPLOYEE_ID = 9999;

1 row deleted.


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INSERT Command

Create one row at a time. Insert many rows from another table.

SQL> INSERT INTO emp VALUES (9999,'Bob','Builder',100,SYSDATE, 10000);

SQL> INSERT INTO emp select employee_id , last_name,first_name,department_id, hire_date, salary FROM HR.employees where department_id =30;

6 rows created.

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2


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UPDATE Command

Use the UPDATE command to change zero or more rows of a table.

SQL> UPDATE EMP SET SALARY= salary +500 Where emp_no =117;

1 row updated.

SQL> UPDATE EMP Set dept_no= (select dept_no from emp where emp_no =117);

6 rows updated.

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DELETE Command

Use the DELETE command to remove zero or more rows

SQL> DELETE FROM emp WHERE emp_no =9000;

0 rows deleted.

SQL> DELETE FROM emp;

6 rows deleted.

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7/33Copyright 2008, Oracle. All rights reserved.9 - 7

MERGE Command

Use the MERGE command to perform both INSERT and UPDATE in a single command.

MERGE INTO EMP a USING

(Select * FROM HR.employees) b ON (a.emp_no =b. employee_id )WHEN MATCHED THEN UPDATE SET a.salary =b.salary WHEN NOT MATCHED THEN INSERT

(emp_no, last_name, first_name, dept_no, hire_date, salary)VALUES

(employee_id,last_name, first_name, department_id,hire_date, salary);



COMMIT and ROLLBACK Commands

To finish a transaction: COMMIT makes the change permanent ROLLBACK undoes the change

SQL> COMMIT;Commit complete.



PL/SQL

Oracles Procedural Language extension to SQL (PL/SQL)is a fourth-generation programming language (4GL). It provides:

Procedural extensions to SQL Portability across platforms and products Higher level of security and data-integrity protection Support for object-oriented programming



Administering PL/SQL Objects

Database administrators should be able to: Identify problem PL/SQL objects Recommend the appropriate use of PL/SQL

Load PL/SQL objects into the database Assist PL/SQL developers in troubleshooting Use supplied packages for administrative and database

maintenance tasks



PL/SQL Objects

There are many types of PL/SQL database objects: Package Package body

Type body Procedure Function Trigger



Functions



Procedures

Are used to perform specific actions Pass values in and out by using an argument list Can be called from within other programs using the

CALL command



Packages

Packages are collections of functions and procedures.Each package should consist of two objects:

Package specification Package body

Package specification



Package Specification and Body



Built-in Packages

Oracle Database comes with several built-in PL/SQLpackages that provide:

Administration and maintenance utilities Extended functionality

Use the DESCRIBE command to view subprograms.

SQL>DESC DBMS_LOCK



Triggers



Triggering Events

Event Type Examples of Events DML INSERT , UPDATE , DELETE

DDL CREATE , DROP , ALTER , GRANT , REVOKE , RENAME

Database LOGON , LOGOFF , STARTUP , SHUTDOWN , SERVERERROR , SUSPEND



Locks

Prevent multiple sessions from changing the same data at the same time

Are automatically obtained at the lowest possible level for a given statement

Do not escalate

Transaction 1

SQL> UPDATE employees2 SET salary=salary*1.13 WHERE employee_id=100;

SQL> UPDATE employees2 SET salary=salary+100 3 WHERE employee_id=100;

Transaction 2



Locking Mechanism

High level of data concurrency: Row-level locks for inserts, updates, and deletes No locks required for queries

Automatic queue management Locks held until the transaction ends (with the COMMIT

or ROLLBACK operation)

Transaction 1

SQL> UPDATE employees2 SET salary=salary*1.13 WHERE employee_id=101;

SQL> UPDATE employees2 SET salary=salary+100 3 WHERE employee_id=100;

Transaction 2



Data Concurrency

Time:

09:00:00

Transaction 1 UPDATE hr.employeesSET salary=salary+100 WHERE employee_id=100;



... ...

Transaction x UPDATE hr.employeesSET salary=salary+100 WHERE employee_id =xxx;



Data and Concurrency../2

Lock modes:ROW SHARE : permits concurrent access to the lockedtable but prohibits sessions from locking the entire table for exclusive access.ROW EXLUSIVE : is the same as ROW SHARE, but also

prohibits locking in SHARE mode. The ROW EXCLUSIVElocks are automatically obtained when updating, inserting, or deleting data. ROW EXCLUSIVE locks allows multiplereaders and one writer.SHARE: permits concurrent queries but prohibits updates tothe locked table. A SHARE lock is required (andautomatically requested) to create an index on a table.However, online index creation requires a ROW SHARElock that is used when building the index.



Data and Concurrency../4

Like any request for a lock, manual lock statements wait until all

sessions that either already have locks or have previously requested locks release their locks.The LOCK command accepts a special argument that controls the waiting behaviour NOWAIT.

NOWAIT returns control to you immediately if the specified table is already locked by another session.It is usually not necessary to manually lock objects.

The automatic locking mechanism provides the data concurrency needed for most applications.

Oracle recommends that you avoid using manual locks, especially when developing applications.Severe performance issues often occur from unnecessarily high locking levels



DML Locks

Each DML transaction must acquire two locks: EXCLUSIVE row lock on the row or rows being updated ROW EXCLUSIVE table-level lock on the table containing

the rows

SQL> UPDATE employees2 SET salary=salary*1.13 WHERE employee_id= 106;

1 row updated.

SQL> UPDATE employees2 SET salary=salary*1.13 WHERE employee_id= 107;

1 row updated.

Transaction 2 Transaction 1



Enqueue Mechanism

The enqueue mechanism keeps track of: Sessions waiting for locks Requested lock mode

Order in which sessions requested the lock



Lock Conflicts

UPDATE employees SET salary=salary+100 WHERE employee_id=100;1 row updated.

9:00:00 UPDATE employees SET salary=salary+100 WHERE employee_id=101;1 row updated.

UPDATE employees SET COMMISION_PCT=2 WHERE employee_id=101;Session waits enqueued due to lock conflict.

9:00:05 SELECT sum(salary) FROM employees;SUM(SALARY)-----------

692634

Session still waiting!

16:30:00

Many selects, inserts, updates,

and deletes during the last 7.5 hours, but no commits or rollbacks!

1 row updated.Session continues.

16:30:01 commit;

Transaction 1 Transaction 2 Time


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Possible Causes of Lock Conflicts

Uncommitted changes Long-running transactions Unnecessarily high locking levels


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Detecting Lock Conflicts

Select Blocking Sessions on the Performance page.

Click the Session ID link to view information about the locking session, including the actual SQL statement.


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Resolving Lock Conflicts

To resolve a lock conflict: Have the session holding the lock commit or roll back Terminate the session holding the lock (in an

emergency)


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Resolving Lock Conflicts with SQL

SQL statements can be used to determine the blocking session and kill it.

SQL> alter system kill session '144,8982' immediate;

SQL> select sid, serial#, username

from v$session where sid in(select blocking_session from v$session)

Result:

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2


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Deadlocks

Transaction 2 Transaction 1

UPDATE employees

SET salary = salary x 1.1WHERE employee_id = 1000;

UPDATE employeesSET salary = salary x 1.1WHERE employee_id = 2000;

ORA-00060:Deadlock detected whilewaiting for resource

UPDATE employees

SET manager = 1342 WHERE employee_id = 2000;

UPDATE employeesSET manager = 1342 WHERE employee_id = 1000;

9:00

9:15

9:16


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Summary

In this lesson, you should have learned how to: Manage data by using SQL Identify and administer PL/SQL objects Describe triggers and triggering events Monitor and resolve locking conflicts


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Practice 9 Overview: Managing Data and Concurrency

This practice covers the following topics: Identifying locking conflicts Resolving locking conflicts

less09 data - managing data and concurrency

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