How It Works: Statistics Sampling for BLOB data

by on January 22, 2009

I worked on an interesting issue today and the outcome deserves a post.  The issue was that when auto update statistics executed the query went from 6 seconds to 6+ minutes. select count(*) from tblImageData where blobData is null Turing on the SP-:StmtStarting and SP:StmtCompleted trace events the internal sample query can be retrieved. SELECT StatMan ( [SC0] ) FROM ( SELECT TOP 100 PERCENT SUBSTRING ( [logo] , 1 , 100 )++ substring ( [blobData] ,           case when datalength ( [blobData] )<= 200  then 101             else datalength ( [blobData] )- 99 end ,             datalength ( [blobData] )) AS [SC0] FROM [dbo] .

The rest is here: 
How It Works: Statistics Sampling for BLOB data

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