Transactions in Oracle Evgenya Kotzeva[1]

8/14/2019 Transactions in Oracle Evgenya Kotzeva[1]

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Transaction controlTransaction controland isolation levelsand isolation levels

in Oraclein OracleEvgeniya Kotzeva

Vereo Technologies

www.vereo.bg

academy.devbg.org www.devbg.org


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ContentsContents

TransactionTransaction controlcontrol

Data Concurrency and ConsistencyData Concurrency and Consistency

in a Multiuser Environmentin a Multiuser Environment

LockingLocking


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Database TransactionDatabase Transaction

A database transaction consists of one ofA database transaction consists of one of

thethe following:following:

DML statements which constitute oneDML statements which constitute one

consistentconsistent change to the datachange to the data

One DDL statementOne DDL statement

One DCL statementOne DCL statement


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Oracle Transaction TypesOracle Transaction Types

Consists of only one DCL

statementData control language

(DCL)

Consists of only one DDL

statementData definition

language

(DDL)

Consists of any number of DML

statements that the Oracle

server treats as a single entity

or a logical unit of work

Data manipulation

language (DML)

DescriptionType


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Transaction boundariesTransaction boundaries

A transaction begins withA transaction begins withthe first executable SQLthe first executable SQL

statement.statement.

A transaction ends withA transaction ends withone of the followingone of the following

events:events:

A COMMIT orA COMMIT or

ROLLBACK statement isROLLBACK statement isissuedissued

A DDL or DCL statementA DDL or DCL statement

executes (automaticexecutes (automatic

commit)commit)

The user exitsThe user exits iiSQL*PlusSQL*Plus

The system crashesThe system crashes


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Advantages of COMMITAdvantages of COMMITand ROLLBACKand ROLLBACK

With COMMIT and ROLLBACK statements, youWith COMMIT and ROLLBACK statements, you

can:can:

Ensure data consistencyEnsure data consistency

Preview data changes before making changesPreview data changes before making changes

permanentpermanent

Group logically related operationsGroup logically related operations


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Controlling transactionControlling transaction


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COMMIT transactionCOMMIT transaction

BeforeBefore COMMITCOMMIT

generated rollbackgenerated rollback

segment records in bufferssegment records in buffers

in the SGAin the SGA

generated redo log entriesgenerated redo log entries

in the redo log buffer of thein the redo log buffer of the

SGA.SGA.

The changes have beenThe changes have beenmade to the databasemade to the database

buffers of the SGA.buffers of the SGA.

AfterAfterCOMMITCOMMIT

The internal transactionThe internal transaction

table for the associatedtable for the associated

rollback segment recordsrollback segment records

updated with SCNupdated with SCN

LGWR writes SGA redoLGWR writes SGA redo

log entries to the onlinelog entries to the online

redo log fileredo log file

Oracle releases locksOracle releases locks

Oracle marks theOracle marks the

transaction complete.transaction complete.


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ROLLBACK transactionROLLBACK transaction

ROLLBACKROLLBACK

Oracle undoes allOracle undoes all

transaction changes usingtransaction changes using

the undo tablespace orthe undo tablespace or

rollback segmentsrollback segments

Oracle releases all theOracle releases all the

transactions locks of datatransactions locks of data

The transaction endsThe transaction ends

ROLLBACK to SAVEPOINTROLLBACK to SAVEPOINT

Oracle rolls back only theOracle rolls back only the

statements run after thestatements run after the

savepoint.savepoint.

Oracle preserves theOracle preserves the

specified savepoint, but allspecified savepoint, but all

savepoints that weresavepoints that were

established after theestablished after the

specified one are lostspecified one are lost

Oracle releases all tableOracle releases all table

and row locks acquiredand row locks acquired

since that savepointsince that savepoint


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State of the DataState of the DataBefore COMMIT or ROLLBACKBefore COMMIT or ROLLBACK

The previous state of the dataThe previous state of the datacan be recoveredcan be recovered..

The current userThe current usercan reviewcan reviewthe results of thethe results of the

DML operations by using the SELECT statement.DML operations by using the SELECT statement.

Other usersOther userscan not viewcan not viewthe results of the DMLthe results of the DML

statements by the current user.statements by the current user.

The affected rowsThe affected rowsare lockedare locked

Other usersOther userscannot changecannot changethe data within thethe data within the

affected rows.affected rows.


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State of the Data after COMMITState of the Data after COMMIT

Data changes are madeData changes are madepermanentpermanentin thein thedatabase.database.

The previous state of the dataThe previous state of the data is permanently lostis permanently lost..

All usersAll users can viewcan viewthe results.the results.

Locks on the affected rowsLocks on the affected rows are releasedare released; those; those

rows are available for other users to manipulate.rows are available for other users to manipulate.

All savepointsAll savepoints are erasedare erased..


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Distributed databaseDistributed database

Distributed transactionDistributed transaction is a transaction thatis a transaction thatincludes one or more statements that update dataincludes one or more statements that update data

on two or more distinct nodes of a distributedon two or more distinct nodes of a distributed

databasedatabase

AA two-phase committwo-phase commit mechanism guaranteesmechanism guarantees thethe

data consistent in all nodes.data consistent in all nodes.


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Autonomous transactionAutonomous transactionss

Autonomous transactions are independentAutonomous transactions are independenttransactions that can be called from within anothertransactions that can be called from within another

transactiontransaction

An autonomous transaction lets you leave theAn autonomous transaction lets you leave thecontext of the calling transactioncontext of the calling transaction

You can call autonomous transactions from withinYou can call autonomous transactions from within

a PL/SQL block by using the pragmaa PL/SQL block by using the pragma

AUTONOMOUS_TRANSACTION.AUTONOMOUS_TRANSACTION.


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Data Concurrency andData Concurrency andConsistencyConsistency

Data concurrencyData concurrency means that many users canmeans that many users can

access data at the same time.access data at the same time.

Data consistencyData consistency means that each user sees ameans that each user sees aconsistent view of the data, including visibleconsistent view of the data, including visible

changes made by the users own transactions andchanges made by the users own transactions and

transactions of other users.transactions of other users.


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The isolation models preventsThe isolation models prevents

Dirty readsDirty reads

Nonrepeatable (fuzzy) readsNonrepeatable (fuzzy) reads

Phantom readsPhantom reads


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Isolation levels (SQL92) controlsIsolation levels (SQL92) controls

PhantomReadNonrepeatableReadDirtyReadIsolationLevel

NNNSerializable

YNNRepeatable read

YYNRead committed

YYYRead

uncommitted


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Oracle isolation levelsOracle isolation levels

The transaction sees only those

changes that were committed at the

time the transaction began and do not

allow any DML statement

Read-only

Serializable transactions see only thosechanges that were committed at the

time the transaction began, plus its own

changes

Serializable

Each query executed by a transactionsees only data that was committed

before the query began (Oracle default

isolation level)

Read committed


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T ti d R dT ti d R d


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Transactions and ReadTransactions and ReadConsistencyConsistency


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Snapshot too oldSnapshot too old

When commit or rollback has been executed,When commit or rollback has been executed,the pre-images can be overwrittenthe pre-images can be overwritten even ifeven if

they are needed to provide a read-they are needed to provide a read-

consistent view to another query.consistent view to another query.

"Snapshot too old" simply means that pre-"Snapshot too old" simply means that pre-

images which the query needs to maintain aimages which the query needs to maintain a

read-consistent view have been overwritten.read-consistent view have been overwritten.


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Common recommendationsCommon recommendations

Common recommendations to reduce the possibility ofCommon recommendations to reduce the possibility of"snapshot too old" are:"snapshot too old" are:

Keep transactions as fast as possibleKeep transactions as fast as possible

Increase the size/number of rollback segmentsIncrease the size/number of rollback segments

DoDo notnotspecify an OPTIMAL size for your rollbackspecify an OPTIMAL size for your rollback

segments.segments.

Increase the size of UNDO_RETENTION parameterIncrease the size of UNDO_RETENTION parameter

(amount of committed undo information to retain in the database)(amount of committed undo information to retain in the database)

Avoid executing long-running queries when transactionsAvoid executing long-running queries when transactions

which update the table are also executing.which update the table are also executing.


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Serializable Transaction FailureSerializable Transaction Failure


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DeadlockDeadlock


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Types of LocksTypes of Locks

Internal locks and latches

protect internal database

structures such as datafiles

Internal locks and latches

DDL locks protect the

structure of schema objects

DDL locks (dictionary locks)

DML locks protect data

For example, table locks lock

entire tables, rowlocks lockselected rows.

DML locks (data locks)

DescriptionLock


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Table LocksTable Locks

RS: row shareRS: row share

RX: row exclusiveRX: row exclusive

S: shareS: share

SRX: share rowSRX: share row

exclusiveexclusive

X: exclusiveX: exclusive


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Data Lock EscalationData Lock Escalation

A transaction holds exclusive row locks forA transaction holds exclusive row locks forall rows inserted, updated, or deleted withinall rows inserted, updated, or deleted within

the transaction. Because row locks arethe transaction. Because row locks are

acquired at the highest degree ofacquired at the highest degree ofrestrictiveness, no lock conversion isrestrictiveness, no lock conversion is

required or performed.required or performed.

Oracle automatically converts a table lockOracle automatically converts a table lockof lower restrictiveness to one of higherof lower restrictiveness to one of higher

restrictiveness as appropriaterestrictiveness as appropriate


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MS SQL Isolation LevelsMS SQL Isolation Levels

SERIALIZABLESNAPSHOT

No EquivalentREAD UNCOMMITTED

No EquivalentREPEATABLE READ

No EquivalentREAD COMMITTED with locking

No EquivalentSERIALIZABLE

READ ONLYSNAPSHOT

READ COMMITTEDREAD COMMITTED with snapshots

SELECT... FOR UPDATESELECT ... WITH (UPDLOCK)

OracleMicrosoft SQL Server 2005


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DB2 Isolation LevelsDB2 Isolation Levels

Isolation levelsIsolation levels

Repeatable Read (RR)Repeatable Read (RR)

Read Stability (RS)Read Stability (RS)

Cursor Stability (CS)Cursor Stability (CS)

Uncommitted Read (UR)Uncommitted Read (UR)

Levels of lockingLevels of locking

TablespaceTablespace

TableTable

RowRow


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Transactions in OracleTransactions in Oracle

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Transactions in Oracle Evgenya Kotzeva[1]

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