Tuesday, August 11, 2009

FAQ : What is String summary statistics in SQL Server?

SQL Server 2005 introduces additional information collected by statistics created on char, varchar, varchar(max), nchar, nvarchar, nvarchar(max), text, and ntext columns. This information, called a string summary, helps the query optimizer estimate the selectivity of query predicates on string patterns . This helps the optimizer better estimate the selectivity of conditions that use the LIKE operator. String summary information is not maintained if the summary for a column sample is larger than the Database Engine can maintain. For example, a string summary will not be maintained on statistics created by using WITH FULLSCAN on a unique varchar(80) column with 80 characters in each string, almost no similarity between strings, on a table with 85,000 rows. To determine whether a string summary is stored for a particular statistics object, use DBCC SHOW_STATISTICS (Transact-SQL) .

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