How Do Indexes Work?
When an index is created, it records the location of values in a table that are associated with the column that is indexed. Entries are added to the index when new data is added to the table. When a query is executed against the database and a condition is specified on a column in the WHERE clause that is indexed, the index is first searched for the values specified in the WHERE clause. If the value is found in the index, the index returns the exact location of the searched data in the table. Figure 16.1 illustrates how an index functions.
Figure 16.1. Table access using an index.
Suppose the following query was issued: ...
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