Re: Re: Stats gather taking longer -19c

Hi,
there was a change in the way bitmap index stats are calculated. Check out Doc ID 2611336.1
Thanks
Lothar
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Von : oracle.developer35_at_gmail.com
Datum : 23/06/2022 - 14:10 (MS)
An : mohamed.houri_at_gmail.com
Cc : oracle-l_at_freelists.org
Betreff : Re: Stats gather taking longer -19c  

  Thank you Mohamed.           

  So basically we must have to remove the hard coded sample and use to auto sample size in all the places. Without this the histogram would be old types and also distinct value calculation would be old types.           

  However we are trying to understand, why it's taking 30minutes more now in 19c even with same 50% sample , which was used in 11.2? Is it because any bug here in 19c?             

    On Thu, 23 Jun 2022, 5:13 pm Mohamed Houri, <     mohamed.houri_at_gmail.com> wrote:                  

     Pap,
     
      
     
     
      Its having histograms on ~36 columns but all of them are still showing height-balanced and frequency. I was expecting those to be hybrid , but its didnt happen, may be because of the way the stats getting gathered on this table i.e. with hard coded 50% sample i.e as below. 
      
     
     
      
     
     
      If you are in 12c and above and you still find histograms of the legacy HEIGHT BALANCED type then it is either that
     
     
      
       
 the statistics for the concerned tables have not yet been calculated in the newer Oracle version
       
or that you are calculating the statistics with a non-default value of the estimate_percent ( sampling percentage):  default value being AUTO_SAMPLE_SIZE 
      
      
       It looks like you are in the second situation since you are using 50% a sample percentage that you should never have done; since the 
       AUTO_SAMPLE_SIZE default value governs,among others, the approximate_ndv algorithm and the new histogram types 
      
     
     
      
     
     
      Best regards
     
     
      Mohamed Houri
     
    
    
    
     
      Le jeu. 23 juin 2022 à 12:23, Pap <
      oracle.developer35_at_gmail.com> a écrit :
      
     
     
      
       Hi, We moved from 11.2.0.4 to version 19.11.0.0.0 of oracle for this database. And we are seeing for one of the table the stats gather has been taking longer. It was used to take ~5-6minutes vs now its taking ~30-40minutes. Dont have enough AWR history to verify the exact run time from ASH/AWR. But we can see it from the application log which notes the start and end time of the stats gather. 
       
        
       
       
        
       
       
        There does exists 5 indexes in this table but when i am seeing , DBA_OPTSTAT_OPERATION_TASKS its showing ~1 minutes for all the index stats gather combinely. Its the table/column gather which takes longer.We are not able to test it as we dont have a 11.2 lower environment with us now though.
       
       
        
       
       
        
       
       
        The table size is 25Gb and is non partitioned one holding ~13million rows. Its having 111 columns. Its having histograms on ~36 columns but all of them are still showing height-balanced and frequency. I was expecting those to be hybrid , but its didnt happen, may be because of the way the stats getting gathered on this table i.e. with hard coded 50% sample i.e as below. 
       
       
        
       
       
        
       
       
        exec DBMS_STATS.GATHER_TABLE_STATS (OWNNAME => NULL, TABNAME => table_name, ESTIMATE_PERCENT => 50, CASCADE => TRUE, method_opt => 'FOR ALL COLUMNS SIZE AUTO', DEGREE => 4);
       
       
        
       
       
        
       
       
        So my question is what must be the reason that same stats gather with ~50% sample size now taking ~30minutes more on 19C as compared to 11.2 where it was finishing in ~5minutes?
       
      
     
    
    
    
     

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http://www.freelists.org/webpage/oracle-l Received on Thu Jun 23 2022 - 15:10:12 CEST