oca 09 - managing data and concurrency

7/29/2019 OCA 09 - Managing Data and Concurrency

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09 - Managing Data and Concurrency

By Muhammad Asghar Khan

Reference: OCA Oracle Database 11g - Admin I Exam Guide by John Watson


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Agenda

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Manage Data Using DML Atomicity

Consistency

Isolation

Durable Execution of DML Statement

Transaction Control

Rollback

Commit EXERCISE 09-1: Manage Data Using DML

Identify and Administer PL/SQL Objects

Procedures and Functions

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Agenda


Packages Database Triggers

Monitor and Resolve Locking Conflicts

Exclusive Lock

Shared Lock

EXERCISE 09-2: Detect and Resolve Lock Contention

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Manage Data Using DML


DML commands change data in tables, they willalso change data in indexes, but this is automatic

RDBMS group one or more DML statements intotransactions

Any RDBMS must be able to pass the ACID test: itmust guarantee atomicity, consistency, isolation,and durability

Atomicity

Principle of atomicity states that either all parts oftransaction must complete, or none of themcomplete

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The rollback of an incomplete transaction is thereversal process

Oracle guarantees atomicity through the use of undo

segments

Consistency

The principle of consistency states that the results of

a query must be consistent with the state of the

database at the time the query started

Through the use of undo segments Oracle guarantees

that if a query succeeds, the result will be consistent

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If your undo segments are incorrectly configured, thequery may not succeed: there is a famous Oracleerror, ORA-1555 snapshot too old, that is raised

Isolation

The principle of isolation states that an incomplete(that is, uncommitted) transaction must be invisibleto the rest of the world

Transaction isolation requires that the database must

conceal transactions in progress from other users Oracle guarantees transaction isolation through use

of undo segments

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Durable

The principle of durability states that once a transactioncompletes, it must be impossible for the database to loseit

Oracle fulfils this requirement through the use of log files(online and archived redo log)

Execution of DML Statement

For any DML operation, it is necessary to work on bothdata blocks and undo blocks, and also to generate redo:

the A, C, and I of the ACID test require generation ofundo; the D requires generation of redo

Redo protects all block changes while undo segment isjust another segment

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Execution of DML statements involve the followingsteps:

1. An empty block or required blocks are checked inthe database buffer cache, or copied into thedatabase buffer cache from the datafiles

2. An empty or expired block of an undo segment isselected

1. For INSERT & UPDATE only a rowid is written to theundo block

2. For a DELETE, the whole row (which might be severalkilobytes) must be written to the undo block

3. Locks are placed on rows and associated index keysthat are going to be affected by the operation

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4. Then the redo is generated

The server process writes to the log buffer the changevectors that are going to be applied to the data blocks

Generation of redo is applied both to table blockchanges and to undo block changes

5. At last the DML statement is carried out in thedatabase buffer cache

Transaction Control

Rollback If anything goes wrong, rollback of transactions in

progress is completely automatic and is carried out bybackground processes

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A manual rollback requires the user to issue the

ROLLBACK command

In the case of an UPDATE, the pre-update versions of the

columns, as stored in the block of undo segment, are used

to construct another UPDATE command that will set the

columns of the row in the table block back to their originalvalues

In case of INSERT Oracle retrieves the rowid of the inserted

row from the undo block and uses it as the key for a DELETE

statement on the table In case of DELETE, Oracle constructs a complete INSERT

statement from the data in the undo block

Then Oracle will issue a COMMIT that will commit both the

original change and the rollback change, as one transaction

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A rollback will itself generate more redo as it executes,

perhaps rather more than the original statement

Commit

A COMMIT involves nothing more than flushing the log

buffer to disk, and flagging the transaction as complete DBWn does absolutely nothing during the commit

When you say COMMIT, LGWR actually does write in real

time: your session will hang until the write is complete

The change vectors written to the redo log are all thechange vectors: those applied to data blocks (tables and

indexes) and those applied to undo segments

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DBWn writes only a few dirty buffers to disk; when acheckpoint is signaled, it writes all dirty buffers todisk

So in principle, your database on disk is corrupted:the datafiles may well be storing uncommitted work,

and be missing committed changes

The COMMIT and ROLLBACK statements does notapply to DDL

When you create a table, you are in fact doing atransaction against data dictionary tables: likeSYS.TAB$ and SYS.COL$ and then the commandconcludes with a COMMIT

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Therefore; executing one or more DML commandsfollowed by a DDL command will commit the wholelot: the DML statements as well as the DDLstatement, this is called auto-commit or implicit-commit

Another situation when auto-commit is happened,On Windows when you exit (EXIT) from a userprocess such as SQL*Plus

This is because built into the SQL*Plus EXITcommand there is a COMMIT statement

If you click in the top-right corner of the SQL*Pluswindow, the transaction will be rollback

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The behavior of SQL*Plus on other platforms maywell be different; the only way to be sure is to test it

There is a SQL*Plus command SET AUTOCOMMIT

ON

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EXERCISE 09-1: Manage Data Using DML


Open two SQL*Plus sessions and connect asSYSTEM

Step 1st Session 2nd Session

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2

Results are the same

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3

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Results differ because transaction isolation conceals the changes

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Results are the same in both sessions

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Oh The DDL statement committed the DELETE, so it cant be rolled back.

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PL/SQL is Oracles proprietary 3GL that runs withinthe database

You can use it to retrieve and manipulate data withSQL, while using procedural constructs such as

IF...THEN...ELSE or FOR or WHILE The PL/SQL code can be stored on a client machine

and sent to the server for execution, or it can bestored within the database as a named block of

code Code stored remotely, or ad hoc code issued at the

SQL*Plus prompt, is anonymous PL/SQL

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Anonymous PL/SQL is less efficient than storedPL/SQL and also causes problems with source code

management, as the code may be distributed

across many machines

Some of the commonly used PL/SQL objects are:

Procedures and Functions

A procedure is a block of code that carries out some

action. It can, optionally, be defined with a number ofarguments

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The arguments can be IN arguments, meaning thatthey are used to pass data into the procedure, or OUTarguments, meaning that they are modified by theprocedure and after execution the new values arepassed out of the procedure

Arguments can also be IN-OUT, where the onevariable serves both purposes

To run a procedure, either call it from within a PL/SQLblock or use the interactive EXECUTE command

A function is similar in concept to a procedure, but itdoes not have OUT arguments and cannot be invokedwith EXECUTE. It returns a single value, with theRETURN statement

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Functions are generally used for relatively simpleoperations

Following code creates and invoke a procedure

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Packages Packages group related procedures and functions

together

A package consists of two objects: a specification and

a body

A package specification lists the functions and

procedures in the package. It can also define variables

and constants accessible to all the procedures and

functions in the package

The package body contains the PL/SQL code that

implements the package

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To create a package specification, use the CREATEPACKAGE command

To invoke a packaged procedure, you must prefix theprocedure name with the package name

Database Triggers

Database triggers are a special category of PL/SQL objectwhich runs (or fires) automatically, when a particularaction is carried out, or a certain situation arises

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Some of the common triggers are; DML triggers, DDL

triggers, Database operation (SERVERERROR,LOGON,

LOGOFF,STARTUP,SHUTDOWN,SUSPEND)

There are numerous uses for triggers:

Auditing users actions Executing complex edits

Security

Enforcing complex constraints

It is impossible to run a trigger by any means other

than its triggering event

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Serialization of concurrent access is accomplishedby record and table locking mechanisms

Locking in an Oracle database is completelyautomatic

Problems only arise if software tries to interferewith the automatic locking mechanism with poorlywritten code

Exclusive Lock

In exclusive lock the first session to request the lockon the row/table gets it, and any other sessionsrequesting write access must wait

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The lock is held until the transaction completes

Shared Lock

Shared lock can be taken on the same object by many

sessions

Shared locks are taken on whole tables, it would not

make any sense to take a shared lock on one row

The purpose of taking a shared lock on a table is to

prevent another session acquiring an exclusive lockon the table

Exclusive locks on tables are only required to execute

DDL statements

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To execute DML on rows, a session must acquire

exclusive locks on the rows to be changed, and shared

locks on the tables containing the rows

The exclusive lock prevents another session from

interfering with the row, and the shared lockprevents another session from changing the table

definition with a DDL statement

Requests for locks are queued Lock contention arises when a session requests a

lock on a row or object and cannot get it

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The causes of lock contention may be the nature ofthe business, a user updates a row and then doesnot commit the change

To detecting and resolve lock contention, Database

home pagePerformance tabAdditionalMonitoring Links sectionInstance Locks link

Blocking locks are the locks that are causingsessions to hang

To terminate a session, either use Database Control,or theALTER SYSTEM KILL SESSIONcommand

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A special case of lock contention is the deadlock Deadlock is a position where two sessions block

each other in such a fashion that both will hang,

each waiting for the other to release its lock

Deadlocks are not the DBAs problem; they are

caused by bad program design and resolved

automatically by the database itself

Information regarding deadlocks is written out tothe alert log, with full details in a trace file

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EXERCISE 09-2: Detect and Resolve Lock

Contention


1.

Using SQL*Plus, connect to database in twosessions as user SYSTEM

2. Create a table to be used for this exercise

3. In your first session, lock all the rows in the

INTEGERS table

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Contention


4.

In your second session, attempt to update a row.The session will hang

5. Connect to your database as user SYSTEM with

database control

6. Navigate to the Performance tab from the

database home page, and then the Database

Locks link in the Additional Monitoring Linkssection

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Contention


7. Observe that the second SYSTEM session is shownas waiting for an EXCLUSIVE lock. Select the radiobutton for the first blocking session and click KillSession

8. In the confirmation window, click Show SQL, ClickReturn and Yes to execute the KILL SESSIONcommand

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Contention

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9.

Returning to your SQL*Plus sessions, you will findthat the second session is now working, but that

the first session can no longer run any commands

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