Analyze seems to work better stats of DBMS

I know this question has been asked thousands of times in different contexts, but this is a very confusing topic:

I have done a lot of googling and analyzed through the docs, but still not found the answer. Recommendation Docs oracle since version 9:


Oracle Corporation strongly recommends that you use the DBMS_STATS package rather than ANALYZE to collect optimizer statistics. This package allows you to collect statistics in parallel, collect global statistics for partitioned objects, and refine your collection of statistics by other means. In addition, possibly cost-based optimizer uses only statistics that have been collected by DBMS_STATS . For more information about this package, see Oracle9i supplied PL/SQL Packages and Types reference .

However, you must use the ANALYZE instruction rather than DBMS_STATS for the collection of statistics that are unrelated to the optimizer based on costs, such as:

  • Use the VALIDATE or LIST CHAINED ROWS clauses
  • To collect information on the blocks not freelist

I'll try to post a test in most case this post earlier, because I can't post the plans exec real reason policies internal, but here's what I've noticed so far:

Oracle 11.2.0.1.0 - Windows Server 2012 R2 64 bit

-Memory settings increased Oracle, enabled WMA and restart of the database;

-'Place' system statistics collected (dbms_stats.gather_system_stats), to let the Oracle knows that he has to work now;

-Statistical collected schema (dbms_stats.gather_schema_stats);

At this point, always kept users have reported performance issues. By examining some queries concerned, I found that there was a lot of full table scan (but yes, all the indexes were available and stats must be updated after stats diagram, please correct me if I'm wrong). So I ran dbms_stats.delete_schema_stats to remember his stats manually in analyse (cannot remember the paper at the moment, but I read, it is not a good idea to mix his stats with DBMS and analyze simultaneously), after after analysis completed, the cost overall reported 125 (before it was 400), but still a few tables running full table scan , so I remove the statistics for a specific table involved in full table scan, we'll call it 'A' and execution plan now opted for table access by index rowid and the cost for this stage went from 30 to 5, but know the overall cost has increased from-800 to 2000...

So my main questions are:

Anyone who works in a specific environment / db analyze where only worked / benefit show?

Can it be dictated by modeling data / design? Whether or not DBMS / analyze will be a better option?

What other steps can I take to isolate the problem?

Is it necessary to clear the shared pool before collecting statistics, as it is sometimes possible to see immediate change to explain the release of the plan?

Suggestions / corrections will be appreciated.

Thank you.

Guilherme, simply upgrade you your database?  I ask because the performance doesn't just normally in all areas except a major problem happens.  You get answers better when the full story is presented.  The main reason I ask is when you move a version of Oracle to an another treating CBO many changes come into play as with 10g on default DBMS_STATS to collect histograms while 9.2 no histograms were collected by default.  The sudden addition of histograms in the equation has burned more than a shop, while others barely noticed.

- -

Back on Oracle 9.2 we had better results using ANALYZE instead of DBMS_STATS. However, we started to use DBMS_STATS with our upgrade to 10.2.  We had only to give some queries immediately after the upgrade.

- -

If you recently upgraded you should be re-assembled all the statistics of objects as part of the upgrade.  Check your current settings of DBMS_STATS.  I recommend that you start with the default values of Oracle in all areas and keep the default values.  If you have any items where your plans get bad, you can change the settings on a basis of object level which is a nice feature because if you want to disable the histograms on a specific object or change the number of buckets used with the object.

- -

If this database has been used some time and performance went all of a sudden jar and then using ANALYZE instead of DBMS_STATS is not the right solution.  In this case, identify any changes to the system that were made as the parameters of database changes, the new application features enabled, etc...

- -

HTH - Mark D Powell.

Tags: Database

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