Are Your Indexes Helping You or Hurting You? Jes Borland, Brent Ozar Unlimited

Transcription

1 Are Your Indexes Helping You or Hurting You? Jes Borland, Brent Ozar Unlimited

2 Your mission: Leave this session able to make better index choices so that your indexes solve performance problems rather than contribute to them.

3 Jes Borland Consultant, Brent Ozar Unlimited

4 Are your indexes helping you or hurting you?

5 What helps? What hurts? Indexes that help Make retrieving data faster Don t slow down INSERT, UPDATE, or DELETE queries significantly Indexes that hurt Aren t being used by SELECT queries Slow down INSERT, UPDATE, or DELETE queries

6 Index Basics

7 This session won t cover Filtered indexes Indexed views XML indexes Spatial indexes Columnstore indexes Memory-optimized indexes

8 A heap of data An unordered collection of data Contains all the columns and rows in the table

9 Apply a clustered index One or more "keys", or columns Data is retrieved in order by that key Remember: the data is not guaranteed to be stored on each page in order! Still contains all the columns and rows in the table

10 Nonclustered indexes Pick one or more columns as the keys Data is retrieved in order by that key All rows are stored, but only a subset of the column(s) are stored This helps! Narrower Less I/O

11 Did you know The nonclustered index contains the clustered index key Thus: no need to put the clustered index key in the nonclustered index

12 Clustered Index Key Size The wider your clustered index key, the wider all of the nonclustered indexes on that table This hurts More storage space More I/O

13 Mission 1: Identify your server s workload

14 Workload types OLTP transactional Many INSERT, UPDATE, DELETE statements Small transactions SELECT a few rows at a time OLAP analytical Many SELECT statements, especially with aggregations Large, long-running transactions Large batch updates

15 What is my server s workload? Dig into the plan cache Plan cache DMVs sp_blitzcache Capture events in real time Extended Events session Server-side trace (especially when aggregated with a tool like Clear Trace)

16 Design for workload OLTP Writes need to be fast Few indexes Narrow indexes OLAP Reads need to be fast Indexes to support queries and aggregations Wide indexes

17 I have both You may need to consider A different isolation level A technology to offload reads

18 Mission 2: Identify what indexes already exist

19 What indexes exist on my tables? Script them out in SSMS Use sys.indexes and sys.index_columns Use a tool like sp_blitzindex

20 Mission 3: Determine if the indexes are helping

21 A table with several nonclustered indexes Same columns, different orders A query executes What will make the query optimizer consider - or not consider - each index? What predicates are in the WHERE clause? How selective are they?

22 Nonclustered index key columns support predicates in the WHERE clause

23 What about SELECT? Use INCLUDE columns These are stored on the page also, but not sorted by the included columns

24 This helps Unless it's all (or most of) the columns from a table Then it hurts

25 What about ORDER BY? Then show how you create an index and turn a query with an expensive Sort into a query with "Ordered=True" on the Index Scan.

26 Lesson learned If your index doesn't contain all the fields the query references, it will have to get them somewhere Seek/key lookup Seek/RID lookup Scan of another index This hurts

27 Mission 4: Identify which query patterns affect index usage

28 Sargability

29 Search this slide Ann Anne Annie Barbara Carrie Danica Danielle Dorothy Elle Esther Fanny Fay Gina Giselle Heather Indica Jeanne Jeannie Jen Jena Jenae Jenelle Jenna Jennie Jennifer Kendra Kiera Kim Lisa Mary Maryann Maryanne Michelle Monica Nina Oprah Pam Paige Patty Quinn Randie Rochelle Sabrina Sandy Sarah Tammy Tessa Uma Victoria Violet Wanda Xena Yasmine Zahara Zabrina

30 How did you find the strings?

31 In SQL Server SELECT FirstName FROM Person WHERE FirstName LIKE '%Jen%'

32 This is a non-sargable query Non Search ARGument -able

33 Try it again

34 Search this slide Ann Anne Annie Barbara Carrie Danica Danielle Dorothy Elle Esther Fanny Fay Gina Giselle Heather Indica Jeanne Jeannie Jen Jena Jenae Jenelle Jenna Jennie Jennifer Kendra Kiera Kim Lisa Mary Maryann Maryanne Michelle Monica Nina Oprah Pam Paige Patty Quinn Randie Rochelle Sabrina Sandy Sarah Tammy Tessa Uma Victoria Violet Wanda Xena Yasmine Zahara Zabrina

35 In SQL Server SELECT FirstName FROM Person WHERE FirstName LIKE 'Jen%'

36 This is a sargable query Search ARGument -able

37 Lesson learned If you have a wildcard contains search, SQL Server will not use your index the way you intended it to Indexes on character fields (in particular) can hurt you

38 Dilemma Sometimes, we need to find a specific value in a field Options?

39 Static value Always occurs in the same place Five characters to the left Last three characters Consider computed column

40 Variable value Full text search Create full-text catalog Create full-text index Do prefix searches Free-text searching tool For truly variable values such as %able% ElasticSearch, Lucene, and more

41 Statistics

42 You are my data

43 The data changes!

44 SQL Server does this, too! Tables and indexes have statistics

45 When are statistics created & updated? Room is empty When the first person enters

46 Room has 400 people There are more than 500 changes One person at a time can leave/enter, but that has to happen 500 times Heck, two people can trade off as long as they do it 500 times

47 Room that has 4,000 people There are 20% changes (4,000 X.2) = 1,300

48 Stadium that has 40,000 people 20% changes (40,000 X.2) = 8,500

49 City that has 4,000,000 people 20% changes (4,000,000 X.2) = = 8,000,500

50 The larger the table The more changes that need to happen for statistics to automatically update Can lead to out-of-date statistics Might need manual intervention

51 Trace flag 2371 SQL Server 2008R2 SP1 + Statistics are updated based on the number of rows in the table Larger tables need fewer updates to trigger auto updates

52 Stats out of date? SQL Server will make bad choices

53 How can I tell? Estimated vs. actual rows in an actual execution plan DBCC SHOWSTATISTICS sys.stats STATS_DATE

54 Lesson learned If statistics are very out of date, SQL Server may choose a bad operation Look at the difference between estimated rows and actual rows if you suspect out-of-date statistics are causing a bad plan

55 Mission 5: Determine if your indexes are being used

56 How do I know if SQL Server is using my indexes? sys.dm_db_index_usage_stats Built into sp_blitzindex But this isn't perfect! Resets when SQL Server is started When database is detached or shut down In 2012+, when index is rebuilt (not reorganized)

57 When can indexes hurt you? Too many on tables frequently updated Not enough key columns; not in right order No included columns; not enough When index exists but query is non-sargable and can't use it When stats for index are out of date and SQL Server can't pick a good plan

58 When can indexes help you? When an appropriate clustered index sorts and stores the data in a useful way When nonclustered indexes are narrow and satisfy WHERE clauses When included columns on nonclustered indexes satisfy SELECT, etc clauses When statistics are up-to-date

59 Are your indexes helping you or hurting you? Chances are good it's both!

60 Know your workload

61 Know what indexes exist

62 Know query patterns that won t use indexes optimally

63 Know which are being used - and which aren't

64 Questions?

65 Thank You for Attending