Having both auto created statistics and index statistics on the same column caused the query optimizer to choose a different — and less than optimal — execution plan than when only the index statistics existed.
From Author : Kendal Van Dyke
http://sqlserverpedia.com/blog/sql-server-2005/tuning-tip-identifying-overlapping-statistics/
WITH autostats ( object_id, stats_id, name, column_id ) AS ( SELECT sys.stats.object_id , sys.stats.stats_id , sys.stats.name , sys.stats_columns.column_id FROM sys.stats INNER JOIN sys.stats_columns ON sys.stats.object_id = sys.stats_columns.object_id AND sys.stats.stats_id = sys.stats_columns.stats_id WHERE sys.stats.auto_created = 1 AND sys.stats_columns.stats_column_id = 1 ) SELECT OBJECT_NAME(sys.stats.object_id) AS [Table] , sys.columns.name AS [Column] , sys.stats.name AS [Overlapped] , autostats.name AS [Overlapping] , 'DROP STATISTICS [' + OBJECT_SCHEMA_NAME(sys.stats.object_id) + '].[' + OBJECT_NAME(sys.stats.object_id) + '].[' + autostats.name + ']' FROM sys.stats INNER JOIN sys.stats_columns ON sys.stats.object_id = sys.stats_columns.object_id AND sys.stats.stats_id = sys.stats_columns.stats_id INNER JOIN autostats ON sys.stats_columns.object_id = autostats.object_id AND sys.stats_columns.column_id = autostats.column_id INNER JOIN sys.columns ON sys.stats.object_id = sys.columns.object_id AND sys.stats_columns.column_id = sys.columns.column_id WHERE sys.stats.auto_created = 0 AND sys.stats_columns.stats_column_id = 1 AND sys.stats_columns.stats_id != autostats.stats_id AND OBJECTPROPERTY(sys.stats.object_id, 'IsMsShipped') = 0
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