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About lndexes

Indcxes arc optional schema objccts that arc associatcd with tablcs. You crcate indexcs on tablcs to improve qucry performance. Just as thc indcx in a guide helps you to quickly locate specific information, an Oracle Databasc index provides quick access to table data.

You can create as many indexcs on a table as you need. You crcate each indcx on one or morę columns of a table. For example, in a purchasc orders table, if you create an index on the vendor number column, then you can sequentially access the rows of the table in vendor number order, without having to actually sort the rows. Additionally, you can directly access all purchase orders issued to a particular vcndor without having to scan the entire table.

After an index is crcated, it is automatically maintained and used by the databasc. Changes to the data or structure of a table, such as adding new rows, updating rows. or deleting rows, are automatically incorporated into all relevant indexes. This is transparent to the u ser.

Some indexes are created implicitly through constraints that are placed on a table. For example, the database automatically creates an index on the columns of a primary key constraint or unique key constraint.

The following topics provide morę background information about indexes:

■Indexes and Performance ■Index Attributes