1. All about Indexes
Gail Shaw

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3. Agenda Why index? Index Architecture Non-clustered Indexes
Other index types

4. Overview

5. Why index?
To reduce the number of rows in consideration for a query as early as possible
To support operations that require a sort order for rows
To enforce uniqueness upon a column, set of columns or subset of values in a column

6. Index Architecture

7. Index architecture B-tree (balanced tree)
Double-linked list at all levels

8. Index architecture

9. Index pages Two types of pages found in indexes Data pages Index pages
Data pages found at the leaf level of a clustered index
Index pages found at the non-leaf level of a clustered index and all levels of a nonclustered index

10. Index operations
Seek
Navigation down the b-tree to find a row or the start/end of a range of rows
Scan
Read of (potentially) the entire index leaf-level
Lookup
Single-row seek against the clustered index/heap

11. Definitions Density – Measure of how unique a column is
Selectivity – What % of a table a predicate will return
Cardinality – How many rows a query operator will affect
Predicate – An expression that evaluates to True or False.
Seek predicate – A predicate which SQL can evaluate by traversing an index’s b-tree

12. Nonclustered Indexes

13. Architecture Nonclustered indexes are secondary structures
They are separate from the table but always in-sync with it
One row in the index for each row in the table
Each index row has a pointer to the actual data row
If the table has a clustered index, this is the clustered index key
If the table is a heap, this is the RID (file ID : page number : slot index)

14. Architecture A, 2 K, 7 A, 2 C, 8 E, 1 E, 8 I, 6 K, 7 M, 6 N, 9 S, 1
F, 7
F, 9
H, 1
I, 6
K, 7
M,6
N,9
S,1
S, 8

15. Guidelines Indexes should be selective or covering (or both)
Wider indexes tend to be more useful
Consider covering indexes for frequently run queries
Consider indexes to support order by and group by
Use INCLUDE for columns that are selected, but not involved in filters/joins

16. Indexing for equality When all where clause predicates are an equality
Order of columns in the index don’t matter for this query
SQL does not seek on one, then the second, etc
SQL will do a multi-column seek to the beginning or end of the range of rows
Order of index columns does matter when trying to support multiple queries with one index
Seek only possible on a left-based subset of the index key
Don’t put most selective column first by default

17. Indexing for inequality
Columns used for equality matches before those for inequality
When dealing with two or more inequality predicates, column order implied by which predicate returns fewer rows
Selectivity much less important for inequality predicates, cardinality is more relevant

18. Indexing for sorts/group by
Indexes are logically ordered by the index key columns
As such, an index can provide the order needed for an ORDER BY or a stream aggregate
Usually should be considered after considering indexes to support the WHERE clause
Not always possible to achieve

19. Indexing for joins Three join types in SQL Server Nested loop
Merge join
Hash join

20. Indexing for joins
Nested loop can benefit from an index on the inner table
Merge requires rows sorted in the join order, so can benefit from indexes on both tables
Hash doesn’t require any indexes at all
Secondary consideration after indexing for the WHERE clause

21. Filtered indexes Index on a subset of the table
Filter condition should be simple. It cannot reference a computed column, spatial data types or Hierarchy.
Very useful for enforcing uniqueness on a subset of the table
To be useful for a query, the predicate must match exactly
Parameterised queries may not use the filtered index

22. Include columns Only included at the leaf level of the index
Does not count towards the 16 column/900 byte limit for index keys
Only TEXT, NTEXT, IMAGE cannot be INCLUDE columns

23. Thank you