Thursday, November 22, 2012

A great query to identify missing indexes and print it as a create index query. They are ordered by a Total Cost
        [Total Cost]  = ROUND(avg_total_user_cost * avg_user_impact * (user_seeks + user_scans),0) 
        , avg_user_impact
        , TableName = statement
        , [EqualityUsage] = equality_columns 
        , [InequalityUsage] = inequality_columns
        , [Include Cloumns] = included_columns
FROM        sys.dm_db_missing_index_groups g 
INNER JOIN    sys.dm_db_missing_index_group_stats s 
       ON s.group_handle = g.index_group_handle 
INNER JOIN    sys.dm_db_missing_index_details d 
       ON d.index_handle = g.index_handle
ORDER BY [Total Cost] DESC;
Where Total Cost stands for:
total cost=(avg_total_user_cost *avg_user_impact *(user_seeks +user_scans))/1000'000
  • avg_total_user_cost – Average cost of the user queries that could be reduced by the index in the group
  • avg_user_impact – Average percentage benefit that user queries could experience if this missing index group was implemented. The value means that the query cost would on average drop by this percentage if this missing index group was implemented
  • user_seeks – Number of seeks caused by user queries that the recommended index in the group could have been used for
  • user_scans – Number of scans caused by user queries that the recommended index in the group could have been used for
Below is a nice query to generate an actual command for an index creation:
PRINT 'Missing Indexes: '
PRINT 'The "improvement_measure" column is an indicator of the (estimated) improvement that might '
PRINT 'be seen if the index was created. This is a unitless number, and has meaning only relative '
PRINT 'the same number for other indexes. The measure is a combination of the avg_total_user_cost, '
PRINT 'avg_user_impact, user_seeks, and user_scans columns in sys.dm_db_missing_index_group_stats.'
PRINT '-- Missing Indexes --'
SELECT CONVERT (varchar, getdate(), 126) AS runtime,
  mig.index_group_handle, mid.index_handle,
  CONVERT (decimal (28,1), migs.avg_total_user_cost * migs.avg_user_impact * (migs.user_seeks + migs.user_scans)) AS improvement_measure,
  'CREATE INDEX missing_index_' + CONVERT (varchar, mig.index_group_handle) + '_' + CONVERT (varchar, mid.index_handle)
  + ' ON ' + mid.statement
  + ' (' + ISNULL (mid.equality_columns,'')
    + CASE WHEN mid.equality_columns IS NOT NULL AND mid.inequality_columns IS NOT NULL THEN ',' ELSE '' END + ISNULL (mid.inequality_columns, '')
  + ')'
  + ISNULL (' INCLUDE (' + mid.included_columns + ')', '') AS create_index_statement,
  migs.*, mid.database_id, mid.[object_id]
FROM sys.dm_db_missing_index_groups mig
INNER JOIN sys.dm_db_missing_index_group_stats migs ON migs.group_handle = mig.index_group_handle
INNER JOIN sys.dm_db_missing_index_details mid ON mig.index_handle = mid.index_handle
WHERE CONVERT (decimal (28,1), migs.avg_total_user_cost * migs.avg_user_impact * (migs.user_seeks + migs.user_scans)) > 10
ORDER BY migs.avg_total_user_cost * migs.avg_user_impact * (migs.user_seeks + migs.user_scans) DESC

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