Thursday, 23 November 2017

Dropping columns statistics after migrating to SQL Server 2014/2016/2017

While migrating databases to new versions of SQL Server, there are some tasks that need to be carried out in order to warrant strong consistent data and physical integrity of databases in tandem with their performance. For instance, it is common to run DBCC CHECKDB, rebuild all indexes, and update columns and index statistics. In some scenarios, we might consider dropping columns statistics and let SQL Server create them again according to new algorithms especially when the new cardinality estimator (CE) is going to be used after upgrading to SQL Server 2014/2016/2017. I personally do that, after restoring databases on the new version of SQL Server I proceed to drop all columns statistics (always having AUTO_CREATE_STATISTICS option enabled) and then SQL Server will definitely create them again based on the nature of database queries and the logic of THE new CE. In a long-term perspective this technique is the most recommendable from my point of view as not only will we have new versions of statistics but also purge old and unused statistics.
Here I will share a script to delete columns statistics. Be caution and only drop them if you have AUTO_CREATE_STATISTICS option enabled, otherwise no statistics will be created and the database performance will be affected tremendously. That is all for now. Let me know any remarks you may have.

SET NOCOUNT ON     
  IF db_name() NOT IN ('model','master','distribution','msdb','tempdb')
    BEGIN 
        DECLARE @schema_name varchar(max)      
        DECLARE @table_name varchar(max)     
        DECLARE @stat_name varchar(max)        
        DECLARE @update_stat_cmd varchar(max)        
        DECLARE @update_stat_msg_header   varchar(max)      
        DECLARE update_stat_cursor CURSOR FOR    
              select  schema_name(o.[schema_id]),  object_name(s1.[object_id]) , s1.name   
              from (  
                select s.[object_id], s.name from sys.stats s  
                left join sys.indexes i on s.name=i.name  
                where i.name is null) s1  
              inner join sys.objects o on o.[object_id]=s1.[object_id]  
              where  o.type='U'  
              order by schema_name(o.[schema_id]),  object_name(s1.[object_id]) , s1.name   
             
         OPEN update_stat_cursor        
         FETCH NEXT FROM update_stat_cursor INTO  @schema_name, @table_name,  @stat_name         
         WHILE (@@fetch_status = 0)        
         BEGIN    
           DECLARE @ini DATETIME, @fin DATETIME     
           SET @update_stat_msg_header =  '->Dropping ['+ RTRIM(@schema_name)  +'].[' + RTRIM(@table_name) + '].[' +@stat_name + ']'      
           PRINT @update_stat_msg_header    
           SET @update_stat_cmd ='DROP STATISTICS ['+ RTRIM(@schema_name)  +'].[' + RTRIM(@table_name) + '].[' +@stat_name + ']'    
           SET @ini=GETDATE()    
           EXEC (@update_stat_cmd)      
           SET @fin=GETDATE()    
          
           FETCH NEXT FROM update_stat_cursor INTO @schema_name, @table_name,  @stat_name            
         END        
     
         PRINT ' '       
         PRINT '----------------------------------------------------------------------------- '        
         SET @update_stat_msg_header = '*************  THERE ARE NO MORE STATISTICS TO BE UPDATED **************'         
         PRINT @update_stat_msg_header        
         PRINT ' '        
         PRINT 'All statistics not linked to any index were rebuilt!'        
          
        CLOSE update_stat_cursor        
        DEALLOCATE update_stat_cursor    
    
   END  
SET NOCOUNT OFF