Analysing Indexes SQLBits 6 th October 2007 © Colin Leversuch-Roberts Kelem Consulting Limited September 2007

Who am I ? © Colin Leversuch-Roberts Kelem Consulting Limited September 2007 Colin Leversuch-Roberts aka Grumpy Old DBA Kelem Consulting Limited

Analysing Indexes How to gather the statistical information about database indexes What to do with it How to use it to your advantage How big is that index Missing Indexes © Colin Leversuch-Roberts Kelem Consulting Limited September

Analysing Indexes WHY ? Documentation Controlled environments Change Requests Change Advisory Board Meetings Urban Myths and would be DBA’s © Colin Leversuch-Roberts Kelem Consulting Limited September

Analysing Indexes No real demo’s My laptop can’t process a 400 million row table live Background ERP System, nearly 8000 indexes Would you believe how this application works? © Colin Leversuch-Roberts Kelem Consulting Limited September

Analysing Indexes Some typical tables © Colin Leversuch-Roberts Kelem Consulting Limited September

Analysing Indexes Some typical indexes All unique ? I think not ! © Colin Leversuch-Roberts Kelem Consulting Limited September

Where to start System views Data not persisted We must capture and store this data Operational cost on indexes ( assuming no RFI ) ** ( No partitioning ) ** 8 © Colin Leversuch-Roberts Kelem Consulting Limited September 2007

Tools of the trade 9 dbo.sysindexesThis is the SQL 2000 system table taken forward as a view. This table may not be supported going forward. sys.dm_db_index_physical_statsThis is actually a function and replaces dbcc showcontig sys.indexes2005 system table contains property information only for each index. sys.dm_db_index_usage_statsSystem view which records the access usage of every index in the database We can view the number of times this index has been used, the type of access and the last access date sys.dm_db_index_operational_statsSystem function which records the operational cost of access to the index We can view information such as the number of pages, locks, latches and waits. sys.objectsContains information on database objects sys.dm_db_missing_index_detailsSystem view which stores information on indexes the optimiser considers are missing. sys.dm_db_missing_index_group_statsSystem view which stores usage and access details for the missing indexes similar to sys.dm_db_index_usage_stats We can also view the improvement that the index is computed to make, this is essentially the same type of output as shown by the Tuning Advisor. sys.dm_db_missing_index_groupsPresumably this view will be of more use in SQL2008 sys.dm_db_missing_index_columnsA system function which returns the columns for a missing index Requires the index handle as a parameter © Colin Leversuch-Roberts Kelem Consulting Limited September 2007

sys.dm_db_index_usage_stats Tells us how the index is used Counts scans, seeks, lookups and writes Values are cumulative, so not so useful Contains last date of of each however 10 © Colin Leversuch-Roberts Kelem Consulting Limited September 2007

sys.dm_db_index_operational_stats Contains a wealth of information Page splits, waits, access at each index level Data is not maintained Not a view Need to snapshot the data © Colin Leversuch-Roberts Kelem Consulting Limited September

Some sample outputs © Colin Leversuch-Roberts Kelem Consulting Limited September 2007

sys.dm_db_index_physical_stats Actually a function Find the size of your index Use with care on large tables Sadly not table valued so difficult to join Roll your own © Colin Leversuch-Roberts Kelem Consulting Limited September

All about granularity Finding unused indexes Dealing with period end processes How often do I collect data and how much Little used indexes Seeks and lookups Making sense of it all 14 © Colin Leversuch-Roberts Kelem Consulting Limited September 2007

Index size A secondary index adds the clustered index Wide clustered indexes make for wide secondary indexes Wide indexes take space Wide indexes produce more io Wide indexes may be considered less cost effective by the optimiser © Colin Leversuch-Roberts Kelem Consulting Limited September

Missing Indexes Also available through the performance dashboard Lots of information Much as you receive from the DTA Simple query to extract basic information Don’t believe it all © Colin Leversuch-Roberts Kelem Consulting Limited September

And to finish Queries to check for duplicated indexes coming in part 5 on my blog Query / Report to show top indexes in cache Query / report to analyse plans in cache © Colin Leversuch-Roberts Kelem Consulting Limited September

More information Coming soon Available for consultancy 18 Thankyou for listening © Colin Leversuch-Roberts Kelem Consulting Limited September 2007