Histograms issue During DB Upgrade.

Question

Hi,
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.If anyone having such experience during migration please help.
When we were in 10g, we were having schemalevel gather stats job with 'METHOD_OPT' as 'AUTO' running each weekend and gathering onle 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 unnecesary histograms and created some on 10g DB. And updated the default schemalevel gather stats job with 'METHOD_OPT' as 'FOR ALL COLUMN SIZE REPEAT'. And we were getting stable enviroment.
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. Need advice on same.
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? Need experts advice on same.
3. Is there any other way to follow, in this scenario? Please advice.

Answer 1

933257 wrote:
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. Need advice on same.
If you're now in production you have a big problem. ANY change you make could make things worse.
If you haven't done it already I would get the plan that techops(Oracle) used to do the migration and compare it with the plan used to when you tested the upgrade to see if there are any differences or (possibly) any gaps. Given that you think the problem is the presence of an unexpected number of histograms, I would guess that the test plan DIDN'T say very much about stats gathering after the upgrade, and that somewhere there should have been a note to ensure that any conflicting data collection jobs in 11g should be modified, or disabled, to match what you were doing in 10g.
In the absence of hard information I would guess that either (a) the automatic overnight stats collection job has not been disabled in 11g, or (b) your weekend task had left a number of parameters in the dbms_stats calls to default and the defaults have changed in the upgrade.
Step 1: find out what has changed in the implementation
Step 2: revert to previous elimination and keep your fingers crossed
Step 3: (optional) - if the stats return to their previous state but you still have performance issues, a short-term fix may be to run with optimizer_features_enable set to its previous value).
Regards
Jonathan Lewis