Performance Tuning :: Histograms During DB Upgrade
Jun 29, 2013
We are currently migrated our database to Oracle 11g i.e 11.2.0.3 (from 10.2.0.3). Now we are facing application slowness on 11g, we found the issue is with missing histogram or some unnecessary histograms in place. Given below are my details.
When we were in 10g, we were having schema level gather stats job with 'METHOD_OPT' as 'AUTO' running each weekend and gathering only STALE stats. And we had faced some performance issue due to optimizer following wrong path of execution due to some missing histograms and some extras too.So we had done skew ness(for each column in table in database) analysis on DB level using 'WIDTH_BUCKET' function manually on each column,and we deleted no of unnecessary histograms and created some on 10g DB. And updated the default schema level gather stats job with 'METHOD_OPT' as 'FOR ALL COLUMN SIZE REPEAT'. And we were getting stable environment.
Now we just migrated and its done by our techops(Oracle) team, i am confused how the histograms has been changed/modified for columns during migration, causing headache now. So now below are my plans and fears associated with them.
1. If i will make the histograms exactly same in both the environments, i.e. will delete histograms which are present in new DB(11g) but were not in old DB(10g).and creating those which were there in old DB(10g) but now not in new DB(11g). My question is , will it be safe to do this or it may impact negatively with 11g optimizers features in place?
2. Or should i try easier option once by making 'METHOD_OPT' for the schema level stats job as 'AUTO' on 11g and try once, as because i believe 11g is more stable and optimizer is equipped with more intelligence in case of stats gathering using AUTO?
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Jul 20, 2013
I've used custom written statistics gathering scripts that by default gather statistics on large tables with a small estimate percentage and FOR ALL COLUMNS SIZE 1. They allow the estimate percentage to be set higher on individual tables and allow me to choose individual columns to have histograms with the maximum number of buckets. The nice thing about this approach was that the statistics ran very efficiently by default and they could be dialed up as needed to tune individual queries. But, with 11g you can set preferences at the table level so that the automatic stats job or even a manual run of gather_table_stats will use your settings. Also, there is some special sampling algorithm associated with auto_sample_size that you miss out on if you manually set the estimate percentage. So, I could change my approach to query tuning and statistics gathering to use AUTO_SAMPLE_SIZE and FOR ALL COLUMNS SIZE AUTO by default and then override these if needed to tune a query or if needed to make the statistics run in an acceptable length of time. I work in an largish company with a team of about 10 DBAs and a bunch of developers and over 100 Oracle databases so we can't really carefully keep track of each system. Having the defaults be less resource intensive saves me the grief of having stats jobs run too long, but it requires intervention when a query needs to be tuned. Also, with my custom scripts I get a lot of hassles from people wanting to know why I'm not using the Oracle default settings so that is a negative. But I've seen the default settings, even on 11.2, run much too long on large tables. Typically our systems start small and then data grows and grows until things start breaking so the auto stats may seem fine at first, but eventually they will start running too long. Also, when the stats jobs run forever or with the automatic jobs don't finish in the window I get complaints about the stats taking too long or not being updated. So, either direction has its pros and cons as far as I can tell.
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Dec 14, 2010
I need to warn readers that I am not a DBA but am heavily involved in application development. Whatever I know about database tuning is whatever I've managed to pick up via self-learning, and I must admit that the sum total of my knowledge isn't a lot.
Anyway, our "DBAs" recently did an upgrade to our 10g database, going from version 10.2.0.2.0 to 10.2.0.4.0. Immediately after the upgrade, a particular query has started to under-perform. The query itself was not altered in any way during the upgrade.
We have two explain plans for the query, a before and an after plan. The two plans are similar but not identical. The plans are too massive to post here, so I hope the following synopsis of the differences will do.
The 10.2.0.2.0 plan:
shows a HASH GROUP BY
has a TempSpc column in the explain plan
shows a particular table (EMP_HISTORY) as having ~1700 rows
The 10.2.0.4.0 plan:
shows SORT GROUP BY instead of HASH GROUP BY does not show a TempSpc column in the explain plan shows the EMP_HISTORY table as having only 25 rows
Other than these points, no other discernible differences can be noted. I'm wondering what would cause HASH to change to SORT. I'm told that stats are up-to-date.
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Jul 12, 2010
Looking to understand the difference between instance tuning and database tuning.
What is the difference between these two tuning exercises? I understand that an instance is memory based structures (logical) where as database consists of physical structures.
However, how does one tune a database the physical structure? Does it have to do with file placements/block sizes etc. Would you agree that a lot of that is taken care by ASM now in 11g? What tools are required/available (third party as well as oracle supplied) for these types of tuning scenarios?
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Oct 31, 2011
I have two tables with 113M records in DWH_BILL_DET & 103M in prd_rerate_chg_que and Im running following merge query, which is running for 13 hrs to update records, which is quiet longer time.
SQL> explain plan for MERGE /*+ parallel (rq, 16) */
INTO DWH_BILL_DET rq
USING (SELECT rated_que_rowid,
detail_rerate_flag_code,
rerate_sel_key,
[code].....
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Sep 30, 2010
How the length of column width effects index performance?
For example if i had IOT table emp_iot with columns:
(id number,
job varchar2(20),
time date,
plan number)
Table key consist of(id, job, time)
Column JOB has fixed list of distinct values ('ANALYST', 'NIGHT_WORKED', etc...).
What performance increase i could expect if in column "job" i would store not names but concrete numbers identifying job names.
For e.g. i would store "1" instead 'ANALYST' and "2" instead 'NIGHT_WORKED'.
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Jun 16, 2010
I have a question about database fragmentation.I know that fragmentation can reduce performance in query times. The blocks are distributed in many extents and scans process takes a long time. Oracle engine have to locate the address of the next extent..
I want to know if there is any system view in which you can check if your table or index has high fragmentation. If it's needed I will have to re-create, move or rebulid the table or index, but before I want to know if the degree of fragmentation is high.
Any useful script or query to do this, any interesting oracle system view?
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Oct 20, 2010
There is a simple way to increase the performance of a query by reducing the row-size of the table it hits. I used it in the past by dividing the table into smaller parts and querying respective smaller table in each query.
what is this method called ? just forgot the method and can't recall it. what this type of row-reduction optimization is called ?
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Jun 16, 2011
How many records could I have in a single table without performance degradation with Standard Edition without partitioning with cutting-edge server (8 or 12 cores, 72 GB RAM, FC 4 Gbit, etc...) and good storage?
300 Millions in only one table with 500K transactions / day is too much?
Simple database with simple schema.
How many records begin to be too many?
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Nov 15, 2010
Testing our 9i to 11g upgrade, we've imported the entire DB into the new machine.We've found that certain procedures are really suffering performance problems. BUT, we've also found, that if we check out a production copy of the procedure from our source code control, and reinstall it, the performance issue goes away. Just alter the procedure and recompiling does NOT work.
The new machine where the 11g database exists is slightly different than the source, but it's not like we have this problem with every procedure. It's only a couple.
any possible reason that we'd have to re-install a procedure to correct a performance problem?
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Apr 12, 2013
I need to check the package performance and need to improve the package performance.
1. how to check the package performance(each and every statement in the package)?
2. In the package using the delete statement to delete all records and observed that delete is taking long time to delete all the records in the table(Table records 7000000). This table is like staging table.Daily need to clean the data before inserting the data into it. what can I use instead of Delete.
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Aug 9, 2010
Somewhere I read that we should not use hints in Oracle production environments, but we can use hints in the development environment and on achieving the desired execution plan we can adjust the 'statistics' to follow that plan without hints.
Q1. If it is true what statistics do we adjust for influencing the execution plan and how?
For example, I have the following simple query:
select e.empid, e.ename, d.dname
from emp e, dept d
where e.deptno=d.deptno;
emp.empid, emp.deptno and dep.deptno columns have indexes and the tables have the standard structure as found in the basic oracle examples.
If I look at the execution plan of the above query then I see that the driving table is empand the driven table is dept.Also the type of join that is taking place is 'Nested Loop'.
Questions: With respect to the above query,
Q 2. If I want to make dept the driving table and emp the driven table then how can I adjust the statistics to achieve that?
Q 3. If I want to use hash join instead of a nested loop join then then how can I adjust the statistics to achieve that?
I can put the ordered and the use_hash hint to effect this but again I have heard that altering statistics is a more robust way to control an execution plan as compared to hints.
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Dec 6, 2011
I have an issue with export(expdp).
When i exporting an user using expdp utility, the load the on the server is going up-to 5. The size of the database is 180GB. Below is the command that i use for export.
expdp sys/xxxx directory=dbpdump dumpfile=expdp_trk_backup.dmp logfile=expdp_trk_backup.log exclude=statistics schemas=trk
Do i need any look into any memory parameters for this?
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May 9, 2012
We have a strange performance issue in our production database.
After migrating our production database from 10g (R2) to 11g (R2) we started seeing major performance issues in the I/O's.
we do have baseline calculation process which runs past midnight every day. Before 11G migration it used to complete in less than 1 Hour, since migration (3 days ago) it started taking more than 5 hours. It just showing major I/O activity (db sequential reads) when the job was running but nothing else. The I/O was always because of the index range scan over a 1.8 billion rows table. By using the Hints we forced Oracle to use the same explain plan it was using on 10G. The query basically index range scans last 60 days worth of data for a specific person out of 1.8 billion rows. Indexes are created with date column in descending order to make the index scans faster while searching for latest data.
when i compared the historical AWR from 10g to 11G I have noticed for same process number of physical reads(sequential) are more in 11G than in 10G. Even physical I/O waits are also more comparatively in 11G because of the more reads i guess. I am thinking may be Oracle scanning index more than what it needs to though we have Histograms on the indexed columns.
My question is , are there any other factors/parameters that we can manipulate to cheat Oracle and make it do fewer physical reads (required) for both both index and table.
Does Histograms only selecting correct index or do they dictating how much percentage of index to be scanned for the current operation also?
Again indexes are same and explain plan is exactly same but more physical I/O in 11G than in 10G. Way to minimize the I/O scans?
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Oct 25, 2012
After upgrade 10g to 11g, the below sql is not working. I have issue with connect by, if we use it with subquery it will hang.
select item_code
from bom_list_pos
where ln_id in (select ln_id
from bom_list_nodes
start with ln_id IN (select ln_id from bom_used_work_pack where rownum =1)
connect by prior ln_id = parent_ln_id)
I ran 10g, able to get it less than minute, but 11g hang. below is explain plan.
11g explain plan
-----------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 19 | 209 | 148M (1)|494:23:18 |
|* 1 | FILTER | | | | | |
| 2 | TABLE ACCESS FULL | BOM_LIST_POS | 810K| 8711K| 7484 (1)| 00:01:30 |
|* 3 | FILTER | | | | | |
|* 4 | CONNECT BY NO FILTERING WITH SW (UNIQUE)| | | | | |
| 5 | TABLE ACCESS FULL | BOM_LIST_NODES | 46041 | 449K| 241 (1)| 00:00:03 |
|* 6 | FILTER | | | | | |
|* 7 | COUNT STOPKEY | | | | | |
| 8 | INDEX FAST FULL SCAN | UWPD_LN_FK_I | 1 | 5 | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter( EXISTS (SELECT 0 FROM "M_SYS"."BOM_LIST_NODES" "SYS_ALIAS_1" WHERE "LN_ID"=:B1 START
WITH EXISTS (<not feasible>)
3 - filter("LN_ID"=:B1)
4 - access("PARENT_LN_ID"=PRIOR "LN_ID")
filter( EXISTS (<not feasible>)
6 - filter("LN_ID"=:B1)
7 - filter(ROWNUM=1)
-------------------
10g explain plan
--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 440 | 9240 | 27 (4)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID | BOM_LIST_POS | 88 | 704 | 6 (0)| 00:00:01 |
| 2 | NESTED LOOPS | | 440 | 9240 | 27 (4)| 00:00:01 |
| 3 | VIEW | VW_NSO_2 | 5 | 65 | 2 (0)| 00:00:01 |
| 4 | HASH UNIQUE | | 5 | 30 | | |
|* 5 | CONNECT BY WITH FILTERING | | | | | |
| 6 | TABLE ACCESS BY INDEX ROWID | BOM_LIST_NODES | | | | |
| 7 | NESTED LOOPS | | 1 | 16 | 1 (0)| 00:00:01 |
| 8 | VIEW | VW_NSO_1 | 1 | 13 | 1 (0)| 00:00:01 |
|* 9 | COUNT STOPKEY | | | | | |
| 10 | INDEX FULL SCAN | UWPD_LN_FK_I | 122 | 366 | 1 (0)| 00:00:01 |
|* 11 | INDEX UNIQUE SCAN | LN_PK | 783 | 2349 | 0 (0)| 00:00:01 |
| 12 | NESTED LOOPS | | | | | |
| 13 | CONNECT BY PUMP | | | | | |
| 14 | TABLE ACCESS BY INDEX ROWID| BOM_LIST_NODES | 5 | 30 | 2 (0)| 00:00:01 |
|* 15 | INDEX RANGE SCAN | LN_LN_FK_I | 5 | | 1 (0)| 00:00:01 |
|* 16 | INDEX RANGE SCAN | LP_LN_FK_I | 88 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("PARENT_LN_ID"=PRIOR "LN_ID")
9 - filter(ROWNUM=1)
11 - access("LN_ID"="$nso_col_1")
15 - access("PARENT_LN_ID"=PRIOR "LN_ID")
16 - access("LN_ID"="$nso_col_1")
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Oct 17, 2011
The following query gets input parameter from the Front End application, which User queries to get Reports.There are many drop down boxes like LOB, FAMILY, BRAND etc., The user may or may not select values from drop down boxes.
If the user select any one or more values ( against each drop down box) it has to fetch all matching values from DB. If the user does'nt select any values it has to fetch all the records, in this case application will send a value 'DEFAULT' (which is not a value in DB ) so that the DB will fetch all the records.
For getting this I wrote a query like below using DECODE, which colleague suggested that will hamper performance.From the below query all the variables V_ are defined in procedure which gets the values selected by user as a comma separated string here V_SELLOB and LOB_DESC is column in DB.
DECODE (V_SELLOB, 'DEFAULT', V_SELLOB, LOB_DESC) IN
OPEN v_refcursor FOR
SELECT /*+ FULL(a) PARALLEL(a, 5) */
*
FROM items a
WHERE a.sku_status = 'A'
[code]...
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Sep 3, 2010
what the principal things to look at when we have for the same query different performance results are?I have 2 different bases: the plan and data are the same but performance results are very differents.
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Mar 17, 2012
are the most important performance keys we have to calculate or take in account to preserve or to increase the DB performance in terms of response times, and whatsoever according to performance ?
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Mar 27, 2013
I am working on an assignement where client is using Oracle 10g but stuck to using RBO Now the application team, from the GUI available to them build dynamic queries and some of them run very slow.
Neither the code can not be changed to tune the queries nor do we get the exact step in the plan which is an issue (being RBO).For some long running queries the Tuning advisor is not producing any recommendations.
Another hurdle is that all the application users are using same application user id so we can not write a logon trigger to use CBO for some particular queries to see what is happening in the background!
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Jul 4, 2012
I want to tuning the next sql sentence. In this sql I want to get the hash_value and sql_text of the sentences that it's causing TX blocks. Is it possible?. This sentence works fine but sometimes It's slow.
SELECT DISTINCT hash_value,
sql_text
FROM gv$sql sq
WHERE hash_value IN (SELECT DISTINCT prev_hash_value
FROM gv$session se
WHERE sid IN (SELECT sid
FROM gv$lock l
WHERE type = 'TX'
AND ctime >= 2000
AND l.inst_id = se.inst_id
AND l.sid = se.sid)
AND sq.inst_id = se.inst_id);
[code]....
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Nov 2, 2012
I see one of my SQL's which is ran by the user on a 10.2.0.3 database changing its SQL_ID after some runs even if the query is not changed a bit! However the HASH VALUE for this query remains the same.
how a same query can have different SQL_ID's but same HASH_VALUE?
Note: Statistics are not modified on the base tables of this query.
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Aug 10, 2011
I am running Oracle 10.2.0.1.0 on MS Windows 2003 server 64-bit with 16G RAM.
Here is the findings for my Oracle database.
SQL> select * * from v$sgainfo;
NAME BYTES RES
-------------------------------- ---------- ---
Fixed SGA Size 1293560 No
Redo Buffers 7094272 No
Buffer Cache Size 830472192 Yes
[code]...
I find that the SGA component "Buffer Cache" is decreasing from the start "1.8G" and down to now 0.8G. On the other hand, the component "Shared Pool" is increasing from the start 0.3G to now 1.2G. I noticed that there are 100 operations of shrinking of "Buffer cache" and growth of "Shared Pool" in Oracle every day.Is it a indicator that I should raise up the SGA_MAX_SIZE?
I tried to increase the SGA_MAX_SIZE to 4G. But I cannot start the Oracle afterward.Is it a limitation of MS Windows(OS) or Oracle?I set the SGA_MAX_SIZE to 3G. This time, I can startup Oracle.What is the optimum/maximum I can set to SGA_MAX_SIZE?Is there any adverse effect/concern when setting the SGA_MAX_SIZE more than 2G?
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Jul 11, 2012
Here i have three tier application. I want to know it host name from sid or sqlid . I want to know which query run on which host. Because i have one user from application to database. So i want to know which query consume more time on which host ?
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Mar 17, 2006
How can I disable redo generation for DML statements.
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Jul 26, 2011
I am getting enq: TS - contention as top event in AWR Report.where can i look into database to fix this issue.
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Aug 14, 2010
I want to run some OLTP benchmarks on my system. I have looked up the TPC-E benchmarking suite .. but the documentation on the site makes no sense to me .
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May 20, 2010
My DB is oracle10g.
I have AWR report comparison for two different days. I want to find the below things.
1. Which day has better performance?
2. What are the top two findings on the report.
I attached the report.
Here are my answer. Please correct me if i am wrong.
1. Which day has better performance? Second day has higher load. Since redosize is showing very high.
2. What are the top two findings on the report.
a) Compared to two days, first day, little bit more I/0 wait for single block read.
b) Compared to two days, 2nd day, it takes higher CPU.
However, which day is best compared to two days?
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Jul 12, 2013
I have installed database in one server. I would like to enable AWR into it. Statistics_level is set to Typical. While running the below script to enable the AWR, its gives error -
SQL> exec dbms_scheduler.enable('GATHER_STATS_JOBS');
BEGIN dbms_scheduler.enable('GATHER_STATS_JOBS'); END;
*
ERROR at line 1:
ORA-27476: "SYS.GATHER_STATS_JOBS" does not exist
ORA-06512: at "SYS.DBMS_ISCHED", line 4343
ORA-06512: at "SYS.DBMS_SCHEDULER", line 2802
ORA-06512: at line 1
make AWR automatical generation.
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Aug 31, 2010
In my awr reports I have CPU time(in seconds) as one of top 5 timed events.Does that mean oracle is waiting for my cpu to execute the statements.But my cpu usage is 0-10% always
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Jun 7, 2012
how to reduce the clustering_factor's value which appears in the user_indexes view?
in my table ,its clustering factor's value is so high:
SQL> SELECT UI.clustering_factor,UI.num_rows,UI.index_type,UI.distinct_keys FROM USER_INDEXES UI WHERE UI.table_name = 'TAWB_AWB';
CLUSTERING_FACTOR NUM_ROWS INDEX_TYPE DISTINCT_KEYS
----------------- ---------- --------------------------- -------------
83609 187603 NORMAL 187603
and its block numbers is 5063
SQL> SELECT COUNT(DISTINCT DBMS_ROWID.rowid_block_number(ROWID)) BLOCK_NUM FROM TAWB_AWB A;
BLOCK_NUM
----------
5063
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