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SQL: How to Find Sessions Generating Lots of Redo or Archive logs (Doc ID
167492.1)
***Checked for relevance on 13-Oct-2015*** goal: How to find sessions
generating lots of redo fact: Oracle Server - Enterprise Edition 8 fact: Oracle
Server - Enterprise Edition 9 fact: Oracle Server - Enterprise Edition 10 fix:
To find sessions generating lots of redo, you can use either of the following
methods. Both methods examine the amount of undo generated. When a transaction
generates undo, it will automatically generate redo as well. The methods are:
1) Query V$SESS_IO. This view contains the column BLOCK_CHANGES which indicates
how much blocks have been changed by the session. High values indicate a
session generating lots of redo. The query you can use is: SQL> SELECT s.sid,
s.serial#, s.username, s.program, 2 i.block_changes 3 FROM v$session s,
v$sess_io i 4 WHERE s.sid = i.sid 5 ORDER BY 5 desc, 1, 2, 3, 4; Run the query
multiple times and examine the delta between each occurrence of BLOCK_CHANGES.
Large deltas indicate high redo generation by the session. 2) Query
V$TRANSACTION. This view contains information about the amount of undo blocks
and undo records accessed by the transaction (as found in the USED_UBLK and
USED_UREC columns). The query you can use is: SQL> SELECT s.sid, s.serial#,
s.username, s.program, 2 t.used_ublk, t.used_urec 3 FROM v$session s,
v$transaction t 4 WHERE s.taddr = t.addr 5 ORDER BY 5 desc, 6 desc, 1, 2, 3, 4;
Run the query multiple times and examine the delta between each occurrence of
USED_UBLK and USED_UREC. Large deltas indicate high redo generation by the
session. You use the first query when you need to check for programs generating
lots of redo when these programs activate more than one transaction. The latter
query can be used to find out which particular transactions are generating redo.
How to identify the causes of High Redo Generation (Doc ID 2265722.1)
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to add to Favorites Customer RecommendedHow to identify the causes of High Redo
Generation (Doc ID 2265722.1) To BottomTo Bottom In this Document Purpose
Troubleshooting Steps References APPLIES TO: Oracle Database - Enterprise
Edition - Version 9.2.0.1 and later Oracle Database Exadata Cloud Machine -
Version N/A and later Oracle Cloud Infrastructure - Database Service - Version
N/A and later Oracle Database Cloud Exadata Service - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Information in this document applies to any platform. PURPOSE Purpose of this
document is to show how to identify the causes of excessive redo generation and
what we can do to mitigate the problem TROUBLESHOOTING STEPS First of all, we
need to remark that high redo generation is always a consequence of certain
activity in the database and it is expected behavior, oracle is optimized for
redo generation and there are no bugs regarding the issue. The main cause of
high redo generation is usually a high DML activity during a certain period of
time and it’s a good practice to first examine modifications on either database
level (parameters, any maintenance operations,…) and application level
(deployment of new application, modification in the code, increase in the
users,..). What we need to examine: 1. Is supplemental logging enabled? The
amount of redo generated when supplemental logging is enabled is quite high
when compared to when supplemental logging is disabled. What Causes High Redo
When Supplemental Logging is Enabled (Doc ID 1349037.1) 2. Are a lot of indexes
being used?, reducing the number of indexes or using the attribute NOLOGGING
will reduce the redo considerably 3. Are all the operation really in need of
the use of LOGGING? From application we can reduce redo by making use of the
clause NOLOGGING. Note that only the following operations can make use of
NOLOGGING mode: - direct load (SQL*Loader) - direct-load INSERT - CREATE TABLE
... AS SELECT - CREATE INDEX - ALTER TABLE ... MOVE PARTITION - ALTER TABLE ...
SPLIT PARTITION - ALTER INDEX ... SPLIT PARTITION - ALTER INDEX ... REBUILD -
ALTER INDEX ... REBUILD PARTITION - INSERT, UPDATE, and DELETE on LOBs in
NOCACHE NOLOGGING mode stored out of line To confirm if the table or index has
"NOLOGGING" set. Issue the following statement. select table_name,logging from
all_tables where table_name = <table name>; -or- select table_name,logging from
all_indexes where index_name = <index name>; 4. Do tables have triggers that
might cause some indirect DML on other tables? 5. Is Auditing enabled the
contributor for this excessive redo generation? 6. Are tablespaces in hot
backup mode? 7. Examine the log switches: select lg.group#,lg.bytes/1024/1024
mb, lg.status, lg.archived,lf.member from v$logfile lf, v$log lg where
lg.group# = lf.group# order by 1, 2; select to_char(first_time,'YYYY-MON-DD')
"Date", to_char(first_time,'DY') day,
to_char(sum(decode(to_char(first_time,'HH24'),'00',1,0)),'999') "00",
to_char(sum(decode(to_char(first_time,'HH24'),'01',1,0)),'999') "01",
to_char(sum(decode(to_char(first_time,'HH24'),'02',1,0)),'999') "02",
to_char(sum(decode(to_char(first_time,'HH24'),'03',1,0)),'999') "03",
to_char(sum(decode(to_char(first_time,'HH24'),'04',1,0)),'999') "04",
to_char(sum(decode(to_char(first_time,'HH24'),'05',1,0)),'999') "05",
to_char(sum(decode(to_char(first_time,'HH24'),'06',1,0)),'999') "06",
to_char(sum(decode(to_char(first_time,'HH24'),'07',1,0)),'999') "07",
to_char(sum(decode(to_char(first_time,'HH24'),'08',1,0)),'999') "08",
to_char(sum(decode(to_char(first_time,'HH24'),'09',1,0)),'999') "09",
to_char(sum(decode(to_char(first_time,'HH24'),'10',1,0)),'999') "10",
to_char(sum(decode(to_char(first_time,'HH24'),'11',1,0)),'999') "11",
to_char(sum(decode(to_char(first_time,'HH24'),'12',1,0)),'999') "12",
to_char(sum(decode(to_char(first_time,'HH24'),'13',1,0)),'999') "13",
to_char(sum(decode(to_char(first_time,'HH24'),'14',1,0)),'999') "14",
to_char(sum(decode(to_char(first_time,'HH24'),'15',1,0)),'999') "15",
to_char(sum(decode(to_char(first_time,'HH24'),'16',1,0)),'999') "16",
to_char(sum(decode(to_char(first_time,'HH24'),'17',1,0)),'999') "17",
to_char(sum(decode(to_char(first_time,'HH24'),'18',1,0)),'999') "18",
to_char(sum(decode(to_char(first_time,'HH24'),'19',1,0)),'999') "19",
to_char(sum(decode(to_char(first_time,'HH24'),'20',1,0)),'999') "20",
to_char(sum(decode(to_char(first_time,'HH24'),'21',1,0)),'999') "21",
to_char(sum(decode(to_char(first_time,'HH24'),'22',1,0)),'999') "22",
to_char(sum(decode(to_char(first_time,'HH24'),'23',1,0)),'999') "23" , count(*)
Total from v$log_history group by to_char(first_time,'YYYY-MON-DD'),
to_char(first_time,'DY') order by
to_date(to_char(first_time,'YYYY-MON-DD'),'YYYY-MON-DD') This will give us an
idea of the times when the high peaks of redo are happening 8. Examine AWR
report: Next step will be examining the AWR from the hour where we have had the
highest number of log switches, and confirm with the redo size that these log
switches are actually caused by a lot of redo generation. In the AWR we can
also see the sql with most of the gets/executions to have an idea of the
activity that is happening in the database and generating redo and we can also
see the segments with the biggest number of block changes and the sessions
performing these changes. Another way to find these sessions is described in
SQL: How to Find Sessions Generating Lots of Redo or Archive logs (Doc ID
167492.1) To find these segments we can also use queries: SELECT
to_char(begin_interval_time,'YY-MM-DD HH24') snap_time, dhso.object_name,
sum(db_block_changes_delta) BLOCK_CHANGED FROM dba_hist_seg_stat dhss,
dba_hist_seg_stat_obj dhso, dba_hist_snapshot dhs WHERE dhs.snap_id =
dhss.snap_id AND dhs.instance_number = dhss.instance_number AND dhss.obj# =
dhso.obj# AND dhss.dataobj# = dhso.dataobj# AND begin_interval_time BETWEEN
to_date('11-01-28 13:00','YY-MM-DD HH24:MI') <<<<<<<<<<<< Need to modify the
time as per the above query where more redo log switch happened (keep it for 1
hour) AND to_date('11-01-28 14:00','YY-MM-DD HH24:MI') <<<<<<<<<<<< Need to
modify the time as per the above query where more redo log switch happened
(interval shld be only 1 hour) GROUP BY to_char(begin_interval_time,'YY-MM-DD
HH24'), dhso.object_name HAVING sum(db_block_changes_delta) > 0 ORDER BY
sum(db_block_changes_delta) desc ; -- Then : What SQL was causing redo log
generation : SELECT to_char(begin_interval_time,'YYYY_MM_DD HH24') WHEN,
dbms_lob.substr(sql_text,4000,1) SQL, dhss.instance_number INST_ID,
dhss.sql_id, executions_delta exec_delta, rows_processed_delta rows_proc_delta
FROM dba_hist_sqlstat dhss, dba_hist_snapshot dhs, dba_hist_sqltext dhst WHERE
upper(dhst.sql_text) LIKE '%<segment_name>%' >>>>>>>>>>>>>>>>>> Update the
segment name as per the result of previous query result AND
ltrim(upper(dhst.sql_text)) NOT LIKE 'SELECT%' AND dhss.snap_id=dhs.snap_id AND
dhss.instance_number=dhs.instance_number AND dhss.sql_id=dhst.sql_id AND
begin_interval_time BETWEEN to_date('11-01-28 13:00','YY-MM-DD HH24:MI')
>>>>>>>>>>>> Update time frame as required AND to_date('11-01-28
14:00','YY-MM-DD HH24:MI') >>>>>>>>>>>> Update time frame as required 9.
Finally, to troubleshoot further the issue and know the exact commands are
being recorded at that particular time frame we can use log miner and mine the
archivelog from the concerned time frame. We can look on v$archived_log and
find the archived log generated at that particular time frame. How To Determine
The Cause Of Lots Of Redo Generation Using LogMiner (Doc ID 300395.1)