Transcript Oracle9i Automatic Undo Management and Flashback Query

Understanding
Oracle9i Automatic Undo
Management
and
Flashback Query
Kirtikumar Deshpande
CLTOUG
July 14, 2005
1
About Me

Senior Oracle DBA

Verizon Information Services

Phone Directories Publication
2
Agenda

Automatic Undo Management

Flashback Query Feature

Demonstration

Q&A
3
New Terminology

Undo Segment, not Rollback Segment

Automatic Undo Management (AUM)


System Managed Undo (SMU)
Manual Undo Management (MUM)

Rollback Undo (RBU)
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AUTOMATIC v/s MANUAL UNDO

Oracle9i database can operate in:

Automatic Undo Management Mode (AUM)



Manual Undo Management Mode (MUM)



Default if database is created using DBCA
Possible only when COMPATIBLE is 9.0.0 and higher
Possible when COMPATIBLE is 9.x, or 8.x
Use it when you are not ready for AUM or you are
upgrading a lower release database
Changing UNDO mode requires instance
startup
5
AUTOMATIC v/s MANUAL UNDO

Manual Undo Management Mode:

Same as using Rollback Segments in Oracle8i
and below

DBA must name, create and manage RBS

Oracle does NOT encourage using this mode
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AUTOMATIC v/s MANUAL UNDO

Automatic Undo Management Mode:


Oracle to name, create, manage Undo
Segments
Oracle to control sizing, number of undo
segments

Requires a new type of tablespace: UNDO

New init.ora parameters
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UNDO Tablespace - Creation

Option in CREATE DATABASE command

CREATE UNDO TABLESPACE command

LMT with SYSTEM policy for space allocation

One Active UNDO tablespace per instance

Each RAC instance has its own UNDO tablespace
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UNDO Tablespace - Creation
CREATE database KED9
controlfile reuse
datafile '/u01/oradata/KED9/system_01.dbf' size 250M
undo tablespace undo_tbs
datafile '/u02/oradata/KED9/undo_tbs_01.dbf' size 500M
logfile
group 1
('/u10/oradata/KED9/redo_g1m1.log') size 25M,
. . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
;
CREATE undo tablespace undo_tbs
datafile '/u02/oradata/KED9/undo_tbs_01.dbf' size 500M;
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UNDO Tablespace

No permanent objects allowed (ORA-30022)

You can change data file size, add data files

You can change data file properties

You can alter tablespace for on-line backups

You cannot offline an active UNDO tablespace

You cannot change extent sizes
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Automatic Undo Segments

Name of AUS
System Generated



_SYSSMUn$ (n is the undo segment number, usn)
Number of AUS





Initially depends on SESSIONS parameter
Minimum required AUS are brought online at startup
More AUS are brought online, or created, as needed,
provided undo space is available
One Transaction per AUS is the desired goal
Two extents per AUS to start with (minextents 2)
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Automatic Undo Segments

Dynamic Extents Transfer


Shrinking Undo Segments



Reusing expired (or unexpired) undo extents from
other undo segments
Every 12 hours SMON shrinks idled undo segments
Foreground processes signal SMON to shrink undo
segments when more undo space is needed
Controlling Use of Undo


Use UNDO_POOL directive in Resource Manager
UNDO quota works similar to tablespace quotas
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Initialization Parameters

COMPATIBLE = 9.0.0 (to use AUM)

UNDO_MANAGEMENT = <auto|manual>

UNDO_TABLESPACE = <ts_name>

UNDO_RETENTION = <seconds|900>

UNDO_SUPPRESS_ERRORS = <false|true>
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UNDO_MANAGEMENT



Auto:
 Oracle deals with undo segments
Manual:
 DBA deals with rollback segments
Not dynamic, instance restart needed when
changed
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UNDO_TABLESPACE


To use at instance startup
(undo_tablespace=<UndoTS>)
If specified Undo TS is not available, any
other available Undo TS is used. If none
present, SYSTEM TS is used with a warning
in alert.log:
***Warning - Executing transaction without active Undo Tablespace
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UNDO_TABLESPACE

When creating new database, if Undo TS is
specified but no ‘undo tablespace’ in
‘CREATE DATABASE’ command, the database
creation fails.
ORA-01092: ORACLE instance terminated. Disconnection forced
ORA-30045: No undo tablespace name specified

Undo TS Can be changed dynamically (not
advisable)
alter system set undo_tablespace = <New Undo TS Name>;
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UNDO_RETENTION






Duration to preserve undo information after
commits
In seconds, defaults to 900 (15 minutes)
Max value is (2³² - 1) seconds
Dynamic at System level
Affects Undo tablespace sizing decision
Not 100% guaranteed
UNDO_RETENTION and adequately sized Undo Tablespace
can minimize occurrence of ORA-1555 error
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UNDO_SUPPRESS_ERRORS

FALSE (default):


TRUE:


Reports as error any manual management
operation related to automatic undo segments
Reports success for all such operations without
actually carrying them out
Dynamic at System and Session level
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New UNDO Views

DBA_UNDO_EXTENTS

Lists the commit times for each extent in the undo
tablespace
(from Oracle9i Database Reference, Release 1)

Describes the extents comprising the segments in all
undo tablespaces in the database
(from Oracle9i Database Reference, Release 2)

V$UNDOSTAT

Statistics for monitoring and tuning Undo space
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DBA_UNDO_EXTENTS
OWNER
SEGMENT_NAME
TABLESPACE_NAME
EXTENT_ID
FILE_ID
BLOCK_ID
BYTES
BLOCKS
RELATIVE_FNO
COMMIT_JTIME
COMMIT_WTIME
STATUS
NOT NULL
NOT NULL
NOT NULL
CHAR(3)
VARCHAR2(30)
VARCHAR2(30)
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
VARCHAR2(20)
VARCHAR2(9)
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DBA_UNDO_EXTENTS





COMMIT_JTIME
 Julian date form
COMMIT_WTIME
 Formatted Wall Clock time
STATUS column to show extent as
 ACTIVE
 UNEXPIRED
 EXPIRED
STATUS may show EXPIRED when you expected
it to be UNEXPIRED
Commit times will return NULLS in 9i R2
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V$UNDOSTAT
BEGIN_TIME
END_TIME
UNDOTSN
UNDOBLKS
TXNCOUNT
MAXQUERYLEN
MAXCONCURRENCY
UNXPSTEALCNT
UNXPBLKRELCNT
UNXPBLKREUCNT
EXPSTEALCNT
EXPBLKRELCNT
EXPBLKREUCNT
SSOLDERRCNT
NOSPACEERRCNT
DATE
DATE
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
NUMBER
----------------
Sample start date/time
Sample end date/time
Last Active Undo TS Number
Undo blocks used
Number of Transactions in sample
MAX Query Length
Max Concurrency
Attempts to steal un-expired blocks
Un-expired blocks released
Un-expired blocks reused
Attempts to steal expired blocks
Expired blocks released
Expired blocks reused
Snapshot Old Error Count
No Space Left Error Count
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V$UNDOSTAT

Available in both SMU and RBU mode.
(From Oracle9i Database Reference Release 1)




Returns null values if using MUM(RBU) mode.
(From Oracle9i Database Reference Release 2)
Returns one useless row in 9i R1, if using MUM
(RBU) mode.
Returns a cumulative number in ‘txncount’ column
in 9i R2. (Bug # 2506744, 3130916)
Reports information in 10 minute intervals

Only when there is a transaction within this interval
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V$UNDOSTAT
BEGIN_TIME
----------------11/08/04 02:32:23
11/08/04 02:22:23
11/08/04 02:12:23
11/08/04 02:02:23
11/08/04 01:52:23
11/08/04 00:32:23
11/08/04 00:22:23
11/08/04 00:02:23
11/07/04 23:52:23
11/07/04 18:12:23
11/07/04 18:02:23
11/07/04 17:52:23
11/07/04 15:52:23
END_TIME
UNDOBLKS
TXNCOUNT
----------------- ---------- ---------11/08/04 02:52:23
0
0
11/08/04 02:32:23
0
206
11/08/04 02:22:23
0
203
11/08/04 02:12:23
0
200
11/08/04 02:02:23
0
195
11/08/04 01:52:23
0
0
11/08/04 00:32:23
1
170
11/08/04 00:22:23
0
0
11/08/04 00:02:23
1
160
11/07/04 23:52:23
0
0
11/07/04 18:12:23
0
54
11/07/04 18:02:23
0
49
11/07/04 17:52:23
0
0
Not a 10 minute interval if TXNCOUNT = 0
TXNCOUNT column is cumulative
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V$UNDOSTAT
B_TIME
----------13:00:27
12:50:27
12:40:27
12:30:27
12:20:27
12:10:27
12:00:27
E_TIME UNDOBLKS TXNCOUNT EXPSTEALCNT EXPBLKRELCNT EXPBLKREUCNT
------------ ---------------- ---------------- ------------------- ------------------------- ------------------------13:10:27
31636
138
27
11776
0
13:00:27
18606
35
5
15864
0
12:50:27
18571
17
3
15864
0
12:40:27
18570
12
1
5112
0
12:30:27
18569
3
0
0
0
12:20:27
9313
2
0
0
0
12:10:27
9269
3
0
0
0
EXPSTEALCNT > 0
=> Dynamic Extent Transfer

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UNDO Tablespace Sizing

Monitor V$UNDOSTAT




Number of Transactions
Number of Undo Blocks consumed
Maximum Query length
Formula:
Undo Space in Bytes = (UR * UDBPS * DB_Block Size)
+ Overhead
UR = Undo Retention Time in Seconds
UDBPS = Undo Blocks used Per Second
Overhead = One DB block for metadata
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Custom View for V$UNDOSTAT
REM – Run as SYS
CREATE OR REPLACE VIEW vw_undostat
AS
SELECT *
FROM v$undostat
WHERE txncount != 0;
CREATE PUBLIC SYNONYM vw_undostat FOR vw_undostat;
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UNDO Tablespace Sizing - 1
SELECT
to_char(min(begin_time),'MM/DD/YYYY HH24:MI:SS') "Begin Time",
to_char(max(end_time),'MM/DD/YYYY HH24:MI:SS') "End Time",
(max(end_time)-min(begin_time))*24*60*60 "Seconds",
sum(undoblks) "UndoBlks",
ceil(sum(undoblks)/((max(end_time)-min(begin_time))*24*60*60)) "UDBPS",
(max(txncount) - min(txncount)) "Xactions",
max(maxquerylen) "MaxQryLen"
FROM
vw_undostat;
Begin Time
End Time
Seconds UndoBlks UDBPS Xactions MaxQryLen
------------------------- ------------------------- ---------- ------------ ----------- ---------- --------------11/07/2004 20:18:15 11/08/2004 21:36:25 91090 693712
8 44393
1973
NOTE: From Oracle9i Release 2 (9.2.0.4) database on AIX 5.2
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UNDO Tablespace Sizing - 2
-- Undo TS Sizing based on Average Undo generation
-- and Max Query Length
SELECT
max(maxquerylen) "MaxQryLen",
ceil(sum(undoblks)/((max(end_time)-min(begin_time))*24*60*60))
* max(maxquerylen) "UndoTSBlocks"
FROM
v$undostat
/
MaxQryLen
-----------------7289
UndoTSBlocks
-----------------29156
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UNDO Tablespace Sizing - 3
-- Undo TS sizing for Current Load and Current Undo
SELECT
rd AS “Retention”,
(rd * (udbps * overhead) + overhead) as "Bytes"
FROM
(SELECT value AS RD FROM v$parameter
WHERE name = 'undo_retention'),
(SELECT (sum (undoblks) /
sum ( ((end_time - begin_time) * 86400))) as UDBPS
FROM v$undostat),
(SELECT value AS OVERHEAD FROM v$parameter
WHERE name = 'db_block_size')
/
Retention
Bytes
----------------- -------------3600
88515698.5
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What is Flashback Query?



Mechanism to view data as it existed at a
point in time in the past
Past data can be viewed as of a timestamp or
System Change Number (SCN)
Using Automatic Undo Management is
strongly recommended
33
What can Flashback Query do?

Recover from accidental data modification

Extract data as of past time (export)

Compare current data with data in the past

Track data changes
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FBQ: How it Works?



Relies on Oracle’s read consistency model
Undo information in undo segments is used to
construct past data
Sufficient Undo information MUST be available
for FBQ to work
35
FBQ: How it Works?



SMON maintains an internal table to map
timestamp to SCN updating it every 5
minutes to record current timestamp and SCN
The internal table (sys.smon_scn_time) can
hold data for up to 5 days (of instance
uptime) and is persistent across startups
SCN is used to reconstruct past data from
Undo segments
36
FBQ: How it Works?

Oracle9i Release 1:


Oracle9i Release 2:


FBQ must be enabled at Session level
Privileges and enhanced SQL syntax can be used
New package: DBMS_FLASHBACK

As SYS, grant execute privilege to user
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DBMS_FLASHBACK
 Procedures:

ENABLE_AT_TIME
exec dbms_flashback.enable_at_time (past_date);
exec dbms_flashback.enable_at_time (to_timestamp
('10-MAR-2002:11:47:00','DD-MON-YYYY:HH24:MI:SS'));

ENABLE_AT_SYSTEM_CHANGE_NUMBER
exec dbms_flashback.enable_at_system_change_number
(23488);

DISABLE
exec dbms_flashback.disable;
38
DBMS_FLASHBACK
 Function:

GET_SYSTEM_CHANGE_NUMBER
SQL> SELECT dbms_flashback.get_system_change_number
2
FROM dual;
GET_SYSTEM_CHANGE_NUMBER
-----------------------------------------------5.98E+12
SQL> set numwidth 18
SQL> /
GET_SYSTEM_CHANGE_NUMBER
-----------------------------------------------5976736332383
39
FBQ : Oracle9i Release 2

DBMS_FLASHBACK Package is still available

No need to enable FB at session level

SQL syntax has a Flashback Clause:
select * from <table>
AS OF <SCN|TIMESTAMP> <expression>
where ……….
40
FBQ : Oracle9i Release 2
select *
from emp as of scn 23478
where emp_id = 100;
select *
from emp as of timestamp sysdate – 1/24;
select * from emp
minus
select * from emp as of timestamp trunc(sysdate);
41
FBQ : Oracle9i Release 2


Object Privilege
grant FLASHBACK on a_table to you;
System Privilege
grant FLASHBACK ANY TABLE to me;
(except data dictionary tables)
NOTE: DBA role has the system privilege
42
FBQ: With Export

Export options

FLASHBACK_SCN

Export Data as of SCN
exp tables=employees flashback_scn = 3853558 file=emp.dmp

FLASHBACK_TIME

Export Data as of TimeStamp
exp tables=test flashback_time=‘”2004-10-14 13:30:00”’
file=test.dmp
43
FBQ: Limitations



SYS cannot use DBMS_FLASHBACK
procedures
Specifying FB time can only find flashback
data to the nearest 5 minute interval
You cannot flashback more than 5 days of
instance uptime
44
FBQ: Limitations


Flashback not possible beyond the time of
DDL operation that changed, or altered,
the table
Not possible for remote table accessed via
DB link
45
Understanding Oracle9i
Automatic Undo Management and
Flashback Query
Demonstration
Q&A
[email protected]
46