chapter 4 indexes. index architecture by default data is inserted on a first-come, first-serve...

Chapter 4

Indexes

Index Architecture By default data is inserted on a first-come,

first-serve basis Indexes bring order to this chaos Once you create an index you will have

data and index pages Indexes can be build against tables or

views SQL Server Query Optimizer determines

how the indexes are used© Wiley Inc. 2006. All Rights Reserved.

Sysindexes Table Contains a record for each index in the

database The index type for each index is stored in

the indid column The FirstIAM column stores the location of

the first Index Allocation Map page• This page tells SQL Server where each

related page is in the database

© Wiley Inc. 2006. All Rights Reserved.

Heaps

A heap is a table with no clustered index Heaps have a value of 0 in the indid column of

sysindexes Extents are not physically next to each other Pages in an extent are See

http://msdn.microsoft.com/en-us/library/ms190969.aspx for pages and extents

SQL Server must read the IAM (Index Allocation Map) page to find each page of the table• This process is called a table scan


http://msdn.microsoft.com/en-us/library/ms190969.aspx


B-tree Format

Indexes are stored in a b-tree format There is a root level at the top Then there are several levels of intermediate

pages The leaf levels hold the actual index data

• The data is different depending on the type of index


Four + types of indexes Clustered Nonclustered Spatial Primary XML Other characters

• Unique • Included columns• Filtered

Clustered Indexes

These physically rearrange the data in your table• Organized much like a dictionary

Have a value of 1 in the indid column of the sysindexes table

The leaf level of the index contains the actual data



Access Data in a Clustered Index

When queried, SQL Server reads the root column of sysindexes• This contains the location of the root page of the index

Each page in the index has a pointer to the next page• Eliminates the need to read an IAM page

SQL Server reads each page until it reaches the data in the leaf level and returns it to the user


Modifying Data in a Clustered Index

Each page has blank space to leave room for new data• Because data must be physically arranged

This space is called the fill factor• A fill factor of 70 means 30 percent free space• A fill factor of 100 or 0 means no room for record on the

page • http://msdn.microsoft.com/en-us/library/ms177459.aspx

New data is inserted in the free space at the end of the correct page


Modifying Data in a Clustered Index, Cont.

Fill factor is not automatically maintained, that is manual

When a page fills completely, half of the data is moved to a new page• This is called a page split

Setting the fill factor correctly can help avoid excessive page splits


?

How many clustered indexes can a table have?

A. One or none

B. Two

C. More than one

D. None of the above

Nonclustered Indexes These do not physically rearrange the data in your

table• Organized much like the appendix at the back of a book

Have a value between 2 and 251 in the indid column of sysindexes• This depends on how many nonclustered indexes you

have on the table

The leaf level of the index contains a pointer to the actual data



Accessing Data in a Nonclustered Index

Like a clustered index, SQL Server reads the root column of sysindexes

Each page in the index has a pointer to the next page

SQL Server reads each page until it reaches the leaf level

Then it uses the pointer in the leaf node to find the data• If you are looking for a range of values, SQL Server

must refer to the index page for each record in the range


Accessing Data in a Nonclustered Index

Nonclustered indexes can be created on tables with a clustered index

If this is the case, the pointer in the leaf level does not point to the data• It points to the key value of the clustered

index


Modifying Data in a Nonclustered Index

In a nonclustered index on a heap, data is inserted wherever there is room

On a clustered index, the data is physically arranged

In both cases, the nonclustered index is updated to point to the new data

This is especially useful when a page split occurs


Costs of indexing

Initial creation • CPU time

Store • The index itself • The related info

Maintain • Update when executing an action query• Page split • Recalculate the statistics

Included Columns and Covering Index

An index that SQL Server can use to satisfy a query without having to access the table is called a covering index

Included columns are not part of the key values of the index• They are included in the data returned to the user• This can speed up data access for complex queries• The columns in the INCLUDE may be too big to be part of the

index

CREATE NONCLUSTERED INDEX IX_PPaR ON Production.ProductReview (ProductID, ReviewerName)

INCLUDE (Comments);


Included Column Considerations Nonkey columns can only be included in nonclustered

indexes. Columns can’t be defined in both the key column and

INCLUDE list. Column names can’t be repeated in the INCLUDE list. At least one key column must be defined. You must have between 1 and 16 key columns defined. You can only have up to a maximum of 1023 included

columns. Nonkey columns can’t be dropped from a table unless the

index is dropped first. The only changes allowed to nonkey columns are:

• Changing nullability (from NULL to NOT NULL and vice versa)• Increasing the length of varbinary, varchar, or nvarchar columns.


Relational Options PAD_INDEX

• Default OFF• When ON, The percentage of free space that is specified by FILLFACTOR is applied

to the intermediate-level pages of the index. FILLFACTOR

• 0 or 100 the same, otherwise the percent used, does not auto re-calculate SORT_IN_TEMPDB

• Where to store some sorting result IGNORE_DUP_KEY STATISTICS_NORECOMPUTE DROP_EXISTING ONLINE (ON and OFF)

• Query and creating of index at the same time? ALLOW_ROW_LOCKS ALLOW_PAGE_LOCKS MAXDOP

• Number of processors


Creating and Maintaining Indexes

Using the GUI tools• Double click a database• Right click the indexes folder• Select “New index” to create a new one• Select the index from the list of indexes to

modify one

chapter 4 indexes. index architecture by default data is inserted on a first-come, first-serve...

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