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Monday, October 01, 2007

The art and science of Oracle DB performance tuning

Back to square one, where I started this blog ? ...

Nooooo ... Here I am not providing a step by step procedure for Oracle performance tuning, however a framework to help a DBA with tackling Oracle performance issues. This could be seen as a reference to understand where to start, what information to collect, where to proceed, how to conclude, etc. .

Without much ado, let's go ahead.

What's the problem ?

Some of us (DBA's) get bad performance notice from users & we might tend to directly jump into trying to find a solution. Instead of trying that, a better way to do that is to first find out what that problem is & how is it seen by the user. First, start with these questions asking the requestor :

1 Is the whole application slow or only some modules are slow ?
2. When did you first notice this problem ?
3. Is the problem noticed all of sudden? Is it gradual ?
4. Is it recurring as you mentioned ?
5. Is there any recent upgrade to application ?
6. Is there any substantial increase in data ?
7. Is there any increase in number of concurrent users ?
8. Is there any increase in total number of users ?
8. Define the problem in your own words.
9. Suppose if we find the certain module of the application is causing the problem, Is it possible to change that module ?
10. Any other comments about the problem ?

PS : I recently got a request to bounce (/ restart) a PRODuction database from a user, stating that the DB was down. When I spoke to him, I discovered that just because he was not able to connect to his application & see anything, he thought the Oracle database behind his application, was down & needed a bounce. I already knew that the database was up & fine, I asked him to speak to the Application team. It was discovered as an application issue, never to be handled by a DBA :-)

Tackle the problem now

Once the problem is clear & the DBA knows that there is a real issue, one may follow these sure-shot steps. You are welcome to follow your own strategy & resolve the issue. If anyone has some strategy to go about performance issues, please share your way of doing things, through comments on this post. I will incorporate them here, as your contribution.

  1. Talk to the user who is facing the problem, to clarify any of the above inputs
  2. Install & run STATSPACK on the concerned Oracle database. By default, spauto.sql schedules the snapshots hourly.
  3. STATSPACK would take some time to provide some useful report. Meanwhile, look at the other aspects below.
  4. If the user knows some areas / schema / SQL / PL/SQL code areas of the database, which might be causing trouble, look at them once. Understand from the user, the database logic used in their "problem operation".
  5. Analyze the database objects involved in the above key areas, for appropriate indexing, fresh statistics.
  6. Look at the explain plan if there's already a SQL code pointed out , otherwise perform this step after you get STATSPACK report.
  7. Start tracing for the schema(s) which the users use to connect to the database.
  8. Have the user run their operation once, to make sure it is captured in STATSPACK & the trace.
  9. Generate the STATSPACK report & use my "STATSPACK post" , the "Survival guide" or a statspack analysis tool to see it in a more understandable format with recommendations too (this tool is not necessary, however, it is helpful).
  10. Format the recent trace (with tkprof utility), generated after the trial of the concerned operation by the user.
  11. Analyze the above 2 reports & find out the issues / areas to be focussed on. STATSPACK report can list out top SQL's / PL/SQL areas of the database. We can start your real, concentrated analysis here. You may repeat steps 4, 5, 6, 7 again if required (most of the times, they will be).
  12. If the problem area is identified as PL/SQL code, it's best to use DBMS_PROFILER to find out the problem line(s) of code. You may use "Oracle official area" to know how to use DBMS_PROFILER or try the database journal (you may just enter DBMS_PROFILER in a search engine & get 100's of results on how to use it). DBMS_PROFILER needs to be installed first as SYS user (profload.sql) & then some objects are created in the concerned schema through schema login (proftab.sql).

Let me know your case-studies - I'll post them here too, along with mine. More on tuning later ...

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