programming-for-dummies

Using Indexes 417
102 John Smith 555-1462
148 Mary Adams 555-7001
152 Nancy Wiggins 555-9604
166 Travis Gullins 555-0147
170 Sam Perkins 555-8136
193 Gail Hall 555-8772
Hash table
A hash table converts data into a numeric
value, which is then used to point to the
actual data.
Figure 2-7:
Comparison
of hash
tables and
indexes.
Adams
Gullins
Hall
Perkins
Smith
Wiggins
Index
Mary 555-7001
Travis 555-0147
Gail 555-8772
Sam 555-8136
John 555-1462
Nancy 555-9604
An index uses part of the data to organize
the rest of the data
Clustered and unclustered indexes
Here are two types of indexes — clustered and unclustered.
A clustered index sorts the actual data. If you have a list of names and
addresses, a clustered index could sort the data by last name, which physically
rearranges the data in order. Because a clustered index physically
rearranges data, you can have only one clustered index per file. Sorting data
by a single field, such as last name, is an example of a clustered index.
Book IV
Chapter 2
An unclustered index doesn’t physically rearrange data but creates pointers
to that data. Because unclustered indexes don’t rearrange the data, you can
have as many unclustered indexes as you need. The drawback of unclustered
indexes is that they’re slower than clustered indexes. A clustered
index finds the data right away whereas an unclustered index needs an extra
step to search the index and then follow the pointer to the actual data, as
shown in Figure 2-8.
Searching
Algorithms