I have created sql tuning task for some query and executed it.After generating report,It recommends me to run the following command:
execute dbms_sqltune.accept_sql_profile(task_name => 'tuning_task_name',
replace => TRUE);
But! Of course I want to know what does this profile do?! and after searching this question on the internet I found the following query:
select
rat.attr1
from
sys.WRI$_ADV_TASKS tsk,
sys.WRI$_ADV_RATIONALE rat
where
tsk.name = 'tuning_task_name'
and rat.task_id = tsk.id
And the result of this query is the following:
I don't understand what does it do :( And also I want to convert this profile hints into the more readable (/+ hint/) sql hints?
Yes, you saw those outline hints.
They will be forced to be used when oracle optimizer parsing this SQL. Hence, the generated execution plan is affected by those hints.
If you want to use similar hints in you SQL, you just need simply copy them into you SQL where follows SELECT, commented by /+/. Be aware the Query Block names, they will be changed in different SQL.
Here is a full list of Oracle SQL hints for your reference.
http://www.hellodba.com/reader.php?ID=208&lang=EN
http://www.hellodba.com/reader.php?ID=220&lang=EN
Hope this helps
Based on page 24 of this PDF it looks like these hints are mostly telling the optimizer to adjust the cardinality estimates for different objects. WHY the profiler is doing this I'm not sure; maybe the statistics are bad or maybe it's making fake statistics to force a better plan? But perhaps a better way to understand the profile is to compare the explain plan before and after.
Related
I have a realy strange problem.
I'm using Oracle 11g.
There is a query executed by Business Objects tool, which the optimizer generate different plans for different users.
When my customer run the BO report it's realy slowly, but when I run it, it's fast.
According to the fact that there is a great plan (take seconds), I tried to enforced the optimizer to use that plan.
The problem is that it's not work.
I tried with baseline and sqlsets but the query used bind variables with different values each time, so it not realy help when the query change.
Is there a way to disable a plan for all sql executions?
This is 1 bad plan.. but can come with a lot of queries becasue of the bind variables.
More, I found in the net information about optimizer_secure_view_merging
parameter that could cause such problem.. but I have few users that got the good plan , not only the owner.. Is that still can be that ?
source:
https://oracledb.wordpress.com/2007/04/10/execution-plans-differents-with-different-users/
If there is another idea what to do..
I'd not call this problem a realy strange.
There is a lot of possible causes that diferent user get different behaviour for the same query.
On trivial cause is to query a non-qualified table.
select * from TAB
This query will access different tables for different users.
The next possibility are different Optimizer Initiation Parameters that could cause that the optimizer for one user may use features that are prohibited for other user.
I'd recommend as a simplest way for troubelshooting to perform the Oracle 10053 trace for both queries.
The trace file contains the complete list of the used paramaters and a simple diff could provide a usefull hint.
If the parameters are not the cause, you'll see in the trace the details why different access paths in the execution plan was taken. (A good introduction to understand 10053 trace is the paper of Wolfgang Breitling I linked above).
i'm struggeling with Performance in oracle. Situation is: Subsystem B has a dblink to master DB A. on System B a query completes after 15 seconds over dblink, db plan uses appropriate indexes.
If same query should fill a table in a stored procedure now, Oracle uses another plan with full scans. whatever i try (hints), i can't get rid of these full scans. that's horrible.
What can i do?
The Oracle Query Optimizer tries 2000 different possibilities and chooses the best one in normal situations. But if you think it choose wrong plan, You may suspect the following cases:
1- Your histograms which belongs to querying tables are deprecated.
2- Your indexes can not be used because of your faulty query.
3- You can use index hints to force the indexes to be used.
4- You can use SQL Advisor or run TKProf for performance analysis and decide what's wrong or what caused bad performance. Check network, Disk I/O values etc.
If you share your query we can give you more information.
Look like we are not taking same queries in two different conditions.
First case is Simple select over dblink & Second case is "insert as select over dblink".
can you please share two queries & execution plans here as You may have them handy. If its not possible to past queries due to security limitations, please past execution plans.
-Abhi
after many tries, I could create a new DB Plan with Enterprise Manager. now it's running perfect.
We got told by dba that our application causes troubles on servers.
There are queries that start like following:
SELECT /* DS_SVC */ /*+ dynamic_sampling(0) no_sql_tune no_monitoring
optimizer_features_enable(default) no_parallel result_cache(snapshot=3600)
OPT_ESTIMATE(#"innerQuery", TABLE, "THIS_#21", SCALE_ROWS=0.0007347778778)
*/ SUM(C1) FROM ...
and they crash server, we receive ORA-12537.
We are using NHibernate, but I am fairly sure those queries are not generated by our application. The queries just have no meaning in business logic, they are some random joins. We don't have sql trace rights, but in logs that dba gives us those queries are executed under our module name.
I googled and found out that DS_SVC is a comment for some service queries that Oracle12 uses in dynamic sampling.
Our queries not exactly complex, couple of left joins with rownum limit 1000.
So the question is - can I say those DS_SVC queries are a problem on dba side? If so, where can I get some docs to prove it?
Looks like a 12c bug. See if changing this helps. Can ask Oracle support as well.
ALTER SESSION SET “_fix_control”=’7452863:0′
https://www.pythian.com/blog/performance-problems-with-dynamic-statistics-in-oracle-12c/
DYNAMIC_SAMPLING hint is used to let CBO collect
cardinality during run time.
Looks like algorithm has been changed in 12c and dynamic sampling is
triggered in a broader set of use cases. This behavior can be disabled
at statement, session or system level using the fix control for the
bug 7452863. For example, ALTER SESSION SET
“_fix_control”=’7452863:0′;
Those queries are generated by the optimizer itself. The feature is called "Dynamic sampling". Until 11g this was by default used only when there were no stats on tables.
Since 12c Dynamic sampling can also be triggered by other new feature "Adaptive execution plans". For example in situations where histograms are missing on columns.
Generally this is quite complex DBA stuff to deal with. There are various ways how to fix "Adaptive exec plans" or to disable them partially/completely.
Best you can do, is to contact Oracle support.
We have added /*+ dynamic_sampling(0) */ hint in our queries. It helped, the exception is gone.
I'm working on a very large query, in a inherited application. This is a large insert-query, that takes 4 tables with well over a million records. I know, I would also rather have this in SQL-server, but there is no infrastructure at this customer to do this :-)
This query has worked for over a year. However, the source-tables keep on growing, and last week it threw the dreaded 'out of system resources'-error. Bummer...!
I think it is possible to optimize this query. Working in MySQL, I would use the explain-command, to see where optimalisation might occur. Is there a equivalent of this in Access? I cannot seem to find it....
kind regards,
Paul
Probably Jet ShowPlan is closest to what you want. You will have to set a registry key. Then query plan information gets dumped to a text file named SHOWPLAN.OUT. You can read about the details in this article on TechRepublic: Use Microsoft Jet's ShowPlan to write more efficient queries
Also try the Performance Analyzer wizard. You can ask it to examine your query alone, or also ask it to examine table or other queries used by that query.
If you haven't compacted the database recently, see whether that improves performance. Compacting also updates index statistics which allows the engine to make better decisions for the query plan.
I have a database(sql server 2005),now there are about 100000 records in the table called users, when I do query use linq to sql, it is very slower and slower.how can I do some operate to improve the speed?
Analyse your query and add some indexes to your table may help.
To get a more specific answer post more specific information (table stucture, indexes you have, the sql code L2S generates, ...)
You could (in order of preference)
Save your query as a stored procedure
Add indexes to your users
table, for what you are querying for/sorting for
Analyze your query
(if it is complicated), see if there's a less-resource-intensive way
of doing it. There are graphical query analyzers to help you.
As a last resort, not use LINQ, but instead ADO.NET Entity Framework, it's significantly faster. But you'll only see performance improvements for crazy stuff, and only if you've already done all of the above.
Use stored procedures and then use linq to sql to get the desired rows, this will give performance.
The best tools at your disposal for analyzing your database access and seeing what needs to be optimized are:
SQL Server Profiler
Graphical Execution Plans
The first one will allow you to see the exact queries being sent to your database from your application, which is especially useful if it turns out that your application is chattier than you think. The second one will allow you to take those queries and see exactly what the SQL server is doing with them.
In the graphical execution plan, look for steps which use a lot of CPU and paths which transfer a lot of records. Those are what you'll want to optimize. It's possible that you're doing a table scan somewhere, which is slow, or maybe joining on many more records than you need somewhere, which is slow, etc.