(SQL) Tip of the Day: Gathering Missing Indexes


Today’s (SQL) Tip…

This week I would like to go through the first few steps I like to send customers to troubleshoot their performance issues in SQL Azure. First and foremost, the easiest win we have is to check out missing indexes. Indexes can have a profound impact on performance in SQL Azure especially if you are seeing a large amount of SOS_SCHEDULER_YIELD waits. My personal favorite article about missing indexes is Bart Duncan’s from 2007: http://blogs.msdn.com/b/bartd/archive/2007/07/19/are-you-using-sql-s-missing-index-dmvs.aspx

The article goes into what the values output mean, but a quick overview is the higher the “improvement_measure” the more it should help your performance. If you see a missing index with a high “user_seeks” value, which is the number of times the index could have been used, you will want to strongly consider adding it.

SELECT

migs.avg_total_user_cost * (migs.avg_user_impact / 100.0) * (migs.user_seeks + migs.user_scans) AS improvement_measure,

'CREATE INDEX [missing_index_' + CONVERT (varchar, mig.index_group_handle) + '_' + CONVERT (varchar, mid.index_handle)

+ '_' + LEFT (PARSENAME(mid.statement, 1), 32) + ']'

+ ' 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 migs.avg_total_user_cost * (migs.avg_user_impact / 100.0) * (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

The query above is the meat of the article and will get you full statements to add missing indexes to your databases!


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