The document discusses transaction control and isolation levels in Oracle databases, detailing the types of transactions (DML, DDL, DCL) and the importance of commit and rollback for data consistency and recovery. It explains different isolation levels, such as read committed and serializable, and introduces concepts like data concurrency, locking mechanisms, and recommendations to avoid issues like 'snapshot too old'. Additionally, it describes the behavior of autonomous transactions and the role of various locks in maintaining data integrity during concurrent access.

1. Transaction control
Transaction control
and isolation levels
and isolation levels
in Oracle
in Oracle
Evgeniya Kotzeva
Vereo Technologies
www.vereo.bg
academy.devbg.org www.devbg.org

2. Contents
Contents
• Transaction
Transaction control
control
• Data Concurrency and Consistency
Data Concurrency and Consistency
in a Multiuser Environment
in a Multiuser Environment
• Locking
Locking

3. Database Transaction
Database Transaction
A database transaction consists of one of
A database transaction consists of one of
the
the following:
following:
• DML statements which constitute one
DML statements which constitute one
consistent
consistent change to the data
change to the data
• One DDL statement
One DDL statement
• One DCL statement
One DCL statement

4. Oracle Transaction Types
Oracle Transaction Types
Type
Type Description
Description
Data manipulation
Data manipulation
language (DML
language (DML)
)
Consists of any number of DML
Consists of any number of DML
statements that the Oracle
statements that the Oracle
server treats as a single entity
server treats as a single entity
or a logical unit of work
or a logical unit of work
Data definition
Data definition
language
language
(DDL)
(DDL)
Consists of only one DDL
Consists of only one DDL
statement
statement
Data control language
Data control language
(DCL)
(DCL)
Consists of only one DCL
Consists of only one DCL
statement
statement

5. Transaction boundaries
Transaction boundaries
A transaction begins with
A transaction begins with
the first executable SQL
the first executable SQL
statement.
statement.
A transaction ends with
A transaction ends with
one of the following
one of the following
events:
events:
• A COMMIT or
A COMMIT or
ROLLBACK statement is
ROLLBACK statement is
issued
issued
• A DDL or DCL statement
A DDL or DCL statement
executes (automatic
executes (automatic
commit)
commit)
• The user exits
The user exits i
iSQL*Plus
SQL*Plus
• The system crashes
The system crashes

6. Advantages of COMMIT
Advantages of COMMIT
and ROLLBACK
and ROLLBACK
With COMMIT and ROLLBACK statements, you
With COMMIT and ROLLBACK statements, you
can:
can:
• Ensure data consistency
Ensure data consistency
• Preview data changes before making changes
Preview data changes before making changes
permanent
permanent
• Group logically related operations
Group logically related operations

7. Controlling transaction
Controlling transaction

8. COMMIT transaction
COMMIT transaction
Before
Before COMMIT
COMMIT
• generated rollback
generated rollback
segment records in buffers
segment records in buffers
in the SGA
in the SGA
• generated redo log entries
generated redo log entries
in the redo log buffer of
in the redo log buffer of
the SGA.
the SGA.
• The changes have been
The changes have been
made to the database
made to the database
buffers of the SGA.
buffers of the SGA.
After
After COMMIT
COMMIT
• The internal transaction
The internal transaction
table for the associated
table for the associated
rollback segment records
rollback segment records
updated with SCN
updated with SCN
• LGWR writes SGA redo
LGWR writes SGA redo
log entries to the online
log entries to the online
redo log file
redo log file
• Oracle releases locks
Oracle releases locks
• Oracle marks the
Oracle marks the
transaction complete.
transaction complete.

9. ROLLBACK transaction
ROLLBACK transaction
ROLLBACK
ROLLBACK
• Oracle undoes all
Oracle undoes all
transaction changes using
transaction changes using
the undo tablespace or
the undo tablespace or
rollback segments
rollback segments
• Oracle releases all the
Oracle releases all the
transaction’s locks of data
transaction’s locks of data
• The transaction ends
The transaction ends
ROLLBACK to SAVEPOINT
ROLLBACK to SAVEPOINT
• Oracle rolls back only the
Oracle rolls back only the
statements run after the
statements run after the
savepoint.
savepoint.
• Oracle preserves the
Oracle preserves the
specified savepoint, but all
specified savepoint, but all
savepoints that were
savepoints that were
established after the
established after the
specified one are lost
specified one are lost
• Oracle releases all table
Oracle releases all table
and row locks acquired
and row locks acquired
since that savepoint
since that savepoint

10. State of the Data
State of the Data
Before COMMIT or ROLLBACK
Before COMMIT or ROLLBACK
• The previous state of the data
The previous state of the data can be recovered
can be recovered.
.
• The current user
The current user can review
can review the results of the
the results of the
DML operations by using the SELECT statement.
DML operations by using the SELECT statement.
• Other users
Other users can not view
can not view the results of the DML
the results of the DML
statements by the current user.
statements by the current user.
• The affected rows
The affected rows are locked
are locked
• Other users
Other users cannot change
cannot change the data within the
the data within the
affected rows.
affected rows.

11. State of the Data after COMMIT
State of the Data after COMMIT
• Data changes are made
Data changes are made permanent
permanent in the
in the
database.
database.
• The previous state of the data
The previous state of the data is permanently lost
is permanently lost.
.
• All users
All users can view
can view the results.
the results.
• Locks on the affected rows
Locks on the affected rows are released
are released; those
; those
rows are available for other users to manipulate.
rows are available for other users to manipulate.
• All savepoints
All savepoints are erased
are erased.
.

12. Distributed database
Distributed database
• Distributed transaction
Distributed transaction is a transaction that
is a transaction that
includes one or more statements that update data
includes one or more statements that update data
on two or more distinct nodes of a distributed
on two or more distinct nodes of a distributed
database
database
• A
A two-phase commit
two-phase commit mechanism guarantees
mechanism guarantees the
the
data consistent in all nodes.
data consistent in all nodes.

13. Autonomous transaction
Autonomous transactions
s
• Autonomous transactions are independent
Autonomous transactions are independent
transactions that can be called from within another
transactions that can be called from within another
transaction
transaction
• An autonomous transaction lets you leave the
An autonomous transaction lets you leave the
context of the calling transaction
context of the calling transaction
• You can call autonomous transactions from within
You can call autonomous transactions from within
a PL/SQL block by using the pragma
a PL/SQL block by using the pragma
AUTONOMOUS_TRANSACTION.
AUTONOMOUS_TRANSACTION.

14. Data Concurrency and
Data Concurrency and
Consistency
Consistency
• Data concurrency
Data concurrency means that many users can
means that many users can
access data at the same time.
access data at the same time.
• Data consistency
Data consistency means that each user sees a
means that each user sees a
consistent view of the data, including visible
consistent view of the data, including visible
changes made by the user’s own transactions and
changes made by the user’s own transactions and
transactions of other users.
transactions of other users.

15. The isolation models prevents
The isolation models prevents
• Dirty reads
Dirty reads
• Nonrepeatable (fuzzy) reads
Nonrepeatable (fuzzy) reads
• Phantom reads
Phantom reads

16. Isolation levels (SQL92) controls
Isolation levels (SQL92) controls
Isolation
Isolation
Level
Level
Dirty
Dirty
Read
Read
Nonrepeatable
Nonrepeatable
Read
Read
Phantom
Phantom
Read
Read
Read
Read
uncommitted
uncommitted
Y
Y Y
Y Y
Y
Read committed
Read committed N
N Y
Y Y
Y
Repeatable read
Repeatable read N
N N
N Y
Y
Serializable
Serializable N
N N
N N
N

17. Oracle isolation levels
Oracle isolation levels
Read committed
Read committed Each query executed by a transaction
Each query executed by a transaction
sees only data that was committed
sees only data that was committed
before the query began (Oracle default
before the query began (Oracle default
isolation level)
isolation level)
Serializable
Serializable Serializable transactions see only those
Serializable transactions see only those
changes that were committed at the
changes that were committed at the
time the transaction began, plus its own
time the transaction began, plus its own
changes
changes
Read-only
Read-only The transaction sees only those
The transaction sees only those
changes that were committed at the
changes that were committed at the
time the transaction began and do not
time the transaction began and do not
allow any DML statement
allow any DML statement

18. Multiversion Concurrency Control
Multiversion Concurrency Control
• Statement-level read consistency
Statement-level read consistency
The data returned by a single query comes from a
The data returned by a single query comes from a
single point in time — the time that the query
single point in time — the time that the query
began
began
• Transaction-level read consistency
Transaction-level read consistency
When a transaction executes in serializable mode,
When a transaction executes in serializable mode,
all data accesses reflect the state of the database
all data accesses reflect the state of the database
as of the time the transaction began
as of the time the transaction began

19. Transactions and Read
Transactions and Read
Consistency
Consistency

20. “
“Snapshot too old”
Snapshot too old”
When commit or rollback has been executed,
When commit or rollback has been executed,
the pre-images can be overwritten
the pre-images can be overwritten even if
even 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 a
images which the query needs to maintain a
read-consistent view have been overwritten.
read-consistent view have been overwritten.

21. Common recommendations
Common recommendations
Common recommendations to reduce the possibility of
Common recommendations to reduce the possibility of
"snapshot too old" are:
"snapshot too old" are:
• Keep transactions as fast as possible
Keep transactions as fast as possible
• Increase the size/number of rollback segments
Increase the size/number of rollback segments
• Do
Do not
not specify an OPTIMAL size for your rollback
specify an OPTIMAL size for your rollback
segments.
segments.
• Increase the size of UNDO_RETENTION parameter
Increase 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 transactions
Avoid executing long-running queries when transactions
which update the table are also executing.
which update the table are also executing.

22. Set the Isolation Level
Set the Isolation Level
You can set the isolation level of a transaction by
You can set the isolation level of a transaction by
using
using one of these statements at the beginning of
one of these statements at the beginning of
a transaction:
a transaction:
• SET TRANSACTION ISOLATION LEVEL
SET TRANSACTION ISOLATION LEVEL
READ COMMITTED;
READ COMMITTED;
• SET TRANSACTION ISOLATION LEVEL
SET TRANSACTION ISOLATION LEVEL
SERIALIZABLE;
SERIALIZABLE;
• SET TRANSACTION ISOLATION LEVEL
SET TRANSACTION ISOLATION LEVEL
READ ONLY;
READ ONLY;

23. Serializable Transaction Failure
Serializable Transaction Failure

24. Modes of Locking
Modes of Locking
• Exclusive lock
Exclusive lock
The mode prevents the associates resource from
The mode prevents the associates resource from
being shared
being shared
• Share lock
Share lock
The mode allows the associated resource to be
The mode allows the associated resource to be
shared, depending on the operations involved
shared, depending on the operations involved

25. Deadlock
Deadlock

26. Types of Locks
Types of Locks
Lock
Lock Description
Description
DML locks
DML locks (data locks)
(data locks) DML locks protect data
DML locks protect data
For example, table locks lock
For example, table locks lock
entire tables, rowlocks lock
entire tables, rowlocks lock
selected rows.
selected rows.
DDL locks
DDL locks (dictionary
(dictionary locks)
locks) DDL locks protect the
DDL locks protect the
structure of schema objects
structure of schema objects
Internal locks
Internal locks and latches
and latches Internal locks and latches
Internal locks and latches
protect internal database
protect internal database
structures such as datafiles
structures such as datafiles

27. Table Locks
Table Locks
• RS: row share
RS: row share
• RX: row exclusive
RX: row exclusive
• S: share
S: share
• SRX: share row
SRX: share row
exclusive
exclusive
• X: exclusive
X: exclusive

28. Data Lock Escalation
Data Lock Escalation
• A transaction holds exclusive row locks for
A transaction holds exclusive row locks for
all rows inserted, updated, or deleted within
all rows inserted, updated, or deleted within
the transaction. Because row locks are
the transaction. Because row locks are
acquired at the highest degree of
acquired at the highest degree of
restrictiveness, no lock conversion is
restrictiveness, no lock conversion is
required or performed.
required or performed.
• Oracle automatically converts a table lock of
Oracle automatically converts a table lock of
lower restrictiveness to one of higher
lower restrictiveness to one of higher
restrictiveness as appropriate
restrictiveness as appropriate

29. MS SQL Isolation Levels
MS SQL Isolation Levels
Microsoft SQL Server 2005
Microsoft SQL Server 2005 Oracle
Oracle
SELECT ... WITH (UPDLOCK)
SELECT ... WITH (UPDLOCK) SELECT... FOR UPDATE
SELECT... FOR UPDATE
READ COMMITTED with
READ COMMITTED with
snapshots
snapshots
READ COMMITTED
READ COMMITTED
SNAPSHOT
SNAPSHOT SERIALIZABLE
SERIALIZABLE
SNAPSHOT
SNAPSHOT READ ONLY
READ ONLY
READ UNCOMMITTED
READ UNCOMMITTED No Equivalent
No Equivalent
READ COMMITTED with locking
READ COMMITTED with locking No Equivalent
No Equivalent
REPEATABLE READ
REPEATABLE READ No Equivalent
No Equivalent
SERIALIZABLE
SERIALIZABLE No Equivalent
No Equivalent

30. DB2 Isolation Levels
DB2 Isolation Levels
Isolation levels
Isolation 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 locking
Levels of locking
• Tablespace
Tablespace
• Table
Table
• Row
Row

31. Transactions in Oracle
Transactions in Oracle
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