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Friday, December 3, 2010

SQL profiles

I have been spending a lot of time with SQL_PROFILES, and figuring out how my profile went stale (and subsequently the performance went from .04 seconds to 100 seconds).


First I know profiles can go bad because of cardinality. Kerry Osborne just did a great blog on this subject, and he talks about how profiles built by the SQL_ADVISOR use opt_estimate.  How to lock SQL_PROFILES created by SQL_ADVISOR. Here is a snippet of what you would see


OPT_ESTIMATE(@"SEL$5DA710D3", INDEX_FILTER, "F"@"SEL$1", IDX$$_1AA260002, SCALE_ROWS=8.883203639e-06)


The issues I've been having come from profiles created by SQLT.  SQLT, can create profiles for you from a previous SQL statement that runs properly.  The profile is named COE_{sql_id}_{plan_hash_value}. 


The first thing I noticed is that I use GTT (global temporary tables) a lot.. In fact during some of the queries that have profiles, the same GTT is called 17 times.  The GTT is primed  with only a single row and dynamic sampling is utilized (there are no stats on the table).  The second thing I noticed is that we have queries, that look simple (a join of 2 objects), but they really call views.  I see the profile is built on a query, not on the views (more later).


So where did I start ?  First I looked at the explain plan for a database that was using the plan I want.  I know that my GTT (tmp_gcc_id) has 1 row in it. 

Operation                                                                          Name                  cardinality
TABLE ACCESS FULL                                                   TMP_GCC_ID      1   

Then later in the plan

NESTED LOOPS                                                                                        179696
     MERGE JOIN CARTESIAN                                                                      8168
         TABLE ACCESS FULL                                         TMP_GCC_ID        8168
           TABLE ACCESS BY INDEX ROWID              order                           1
              INDEX RANGE SCAN                                    idx_order                  22


The  interesting thing is that the cardinality is 8168 for the GTT for the rest of Explain plan.. This dramitically inflluences the cost, and cardinality estimates throughout.  Oracle however does use the profile and keeps the plan to be what I want it to be.  Now I look at the database where the profile is causing performance issues.

Operation                                                                    Name                  cardinality
TABLE ACCESS  FULL                                       TMP_GCC_ID         1




then later in the plan



As far as ADG (active data guard), baselines can't be created. They require a sql_tuning_set which gets created in the database.


Baselines have long way to go to make them as portable as profiles, and profiles have a long way to go to make them as stable as baselines.  I hope there is migration path from profiles to baselines, but I haven't heard any rumblings yet.
NESTED LOOPS                                                                                        238,125
     MERGE JOIN CARTESIAN                                                                      8168
         TABLE ACCESS FULL                                         TMP_GCC_ID        8168
           TABLE ACCESS FULL                                     ORDER            534,285

    


I
think my problem with the profile causing a FTS on a large table is 2 fold.


GTT  -- SInce I am using a GTT, with dynamic sampling, oracle is only using dynamic sampling the first time it encounters the GTT in the query (from what I can gather).. All the subsequent subselects utilizing the GTT default to a cardinality of 8168, and follow that through..
This works fine when the profile is recognized, and working.. Once the profile starts to fail, the bad cardinality drives FTS where they shouldn't be.

PROFILES on Queries with Underlying views -  My query that I built the profile on has an underlying view that does most of the work.. The view was modified, and the order of the tables accessed was changed.  This was enough to throw off the profile.. The profile is built on the SEL$ names that oracle assigns to each section of the query.  Since the query didn't change, the profile was used.  And since the SEL$ names no longer matched the new view, the profile couldn't be properly utilized.  Since the profile wasn't properly utilzed, oracle defaulted to figuring out the best plan.. GTT's cause issues because they didn't dynamic sample every time.  The default cardinality of the GTT (8168) was enough to cause full table scans.

The moral of the story is that profiles can go bad 2 ways.

1) if they use any sort of cardinality hint (like Kerry mentions).

2) You change an underlying object (like a view), that doesn't change the query text.

Now the next topic is Baselines.  I tried to use baselines (to save myself all this trouble with profiles), and I had no luck.. We have multiple databases with the same schema, but different login ID's.  In order to utilize baselines you have to create SQL_TUNING_SETS.   Creating a SQL_TUNING_SET is much more difficult than just creating a SQL_PROFILE from an existing plan.  The other issue I have had is that we already have profiles on our queries that cause us trouble.. Creating a baselines, relies on the profile.. Create a baseline, remove the profile, and the baseline fails.,. This dependency to the profile makes it very difficult to replace our current profiles.

Lastly baselines have a column called "parsing_schema_name".. I love the idea that you can move baselines between databases (and promote them as part of your code), but we don't have the same parsing_schema_name in all our database.. I try to import the baseline, and it fails because the parsing_schema_name doesn't exist in the database.

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