1. Indexing For Optimal Performance
Robert Douglas
SQL Saturday #464, Melbourne
20th February 2016

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I attack SQL challenges by dropping onto them from above.

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5. Robert Douglas Consultant for SQL Services Limited
BA(hons),MCTS, MCITP(2005/2008), MCSA(2012)
Consultant for SQL Services Limited
Christchurch\Nelson SQL Server Users Group

6. What We Will Cover What is an Index and why should I care?
Fragmentation
What is it?
Why does it effect my queries?
How to combat it?
Designing Smarter Indexes
Indexing Mistakes to Avoid

7. Types of Indexes Heaps Clustered Index One per table The actual data
Non-Clustered Index
Many per table
Copies ‘bits’ of the table
Covering Index?

8. What is an Index and Why Should I Care?
PAGE HEADER
DATA PAGE
DATA 8K
DATA
(Index OR
Data Rows)
SLOT ARRAY

9. What is an Index and Why Should I Care?
DATA 8K
Index Pages
DATA 8K
DATA 8K
DATA 8K
DATA 8K
DATA 8K
DATA 8K
DATA 8K
DATA 8K
DATA 8K
DATA 8K
DATA 8K
DATA 8K

10. What is an Index and Why Should I Care?
A-J
A - E
F-J A B C D E F G H I J

11. What is an Index and Why Should I Care?
A-J
A - E
F-J A B C D E F G H I J

12. SELECT * FROM dbo.TABLE WHERE Word=‘Go’
A-J
A - E
F-J A B C D E F G H I J

13. SELECT * FROM dbo.TABLE WHERE Word BETWEEN ‘B’ AND ‘G’
A-J
A - E
F-J A B C D E F G H I J

14. SELECT * FROM dbo.TABLE WHERE Word BETWEEN ‘B’ AND ‘G’ READAHEAD
A-J
A - B
C-D
E-F
G-H
I-J A B C D E F G H I J

15. What is Fragmentation? External Fragmentation
Referred to as “Logical Fragmentation” in books online.
Next logical page is not the next physical page.
External Fragmentation reduces Read Ahead Performance.
That reduces the efficiency of your range scans.
Only refers to pages on disk(pages in memory are always logically contiguous)
avg_fragmentation_in_percent in the sys.dm_db_index_physical_stats DMF

16. What is Fragmentation? Internal Fragmentation
Sometimes referred to as physical fragmentation.
Empty space on pages.
Avg_page_space_used_in_percent in sys.dm_db_index_physical_stats DMF.
The effects of internal fragmentation are:
More disk space needed to hold the data.
More IO to read the same amount of data.
More space in memory used to hold the same data.

17. What Causes Fragmentation? Page Splits
Page Splits happen when there is not enough space on a page.
SQL has to write in as specific location.
Inserts on pages that are already full.
Updates which increase the size of a row.
Snapshot isolation(makes updated records 14 bytes longer).
Page must either be compacted or split.
Close to, but not always 50/50.

18. What happens when you discover fire?
F-J
FIRE F G H I J

19. What happens when you discover fire?
F-J
FIRE G H I J F F2

20. What happens when you discover fire? A “Good” Page Split
A-F
FIRE A B C D E F F2

21. How Do I Remove Fragmentation? REBUILD and REORGANISE Indexes
Rebuild Indexes
This is an expensive operation
It requires the size of the index as additional space
It reads and writes the entire index.
It takes locks on your pages.
Reorganise Indexes
Always online
Rollback is one page.
Doesn’t update stats.
Maintenance Plans.
Smart scripts

22. How Do I Remove Fragmentation? Avoid it in the first place
Re-Architect your tables
Avoid GUIDS or other random keys on highly transactional tables.
Be aware of when updates are likely to occur on variable length columns.
Keep clustered index narrow if possible.
Avoid very wide rows
Fill Factor
Select * from sys.configurations where name like 'fill factor%‘
select * from sys.indexes order by fill_factor DESC
Avoid database shrinks.

23. How Shrinking Works

24. Designing Smarter Indexes
Know your workload
Query Hints
Missing Indexes DMV
Sys.dm_db_missing_index_details
SYS.dm_db_missing_index_Columns
SYS.dm_db_missing_index_groups
SYS.dm_db_missing_index_group_stats
Prune unwanted Indexes
Unused Indexes
SP_BlitzIndex – BrentOzar.com

25. Indexing Mistakes to Avoid #1: Blindly Following “Suggested Indexes” Recommendations
Query Plan Hints
Missing Indexes DMV
Sys.dm_db_missing_index_details
SYS.dm_db_missing_index_Columns
SYS.dm_db_missing_index_groups
SYS.dm_db_missing_index_group_stats
Database Tuning Adviser
3rd Party Tools
The Internet

26. Indexing Mistakes to Avoid #2: Fill Factor Suicide
Fill factor will remove or slow External Fragmentation
But it increases internal fragmentation.

27. Indexing Mistakes to Avoid #3: The Wrong Maintenance
If you have regular maintenance in place – do you know what it does?
Rebuilding Indexes that don’t need it.
Reorganising Indexes without updating stats

28. Bonus Indexing Mistake: Readable Secondaries
If you have a readable secondary (Mirroring or AOAG) it turns on snapshot isolation on the secondary replica. That doesn’t turn on snapshot isolation on the primary, but it will add the 14 byte versioning tag on them. If your pages are built to 100%, this will cause every page you query on the secondary split on the primary.

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