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You can create an index (also called secondary key) to make it quicker for the database system to find data records in a table. An index is a database object that can be created for an individual column or a series of columns in a database table. An index consists of one or more inversion lists.

An index name must be unique when combined with the table name. When defining an index, you determine whether the column values of different rows in the indexed columns have to be unique or not.

Indexes

Property

Description (Link to SQL Reference Manual)

Index name

Index Name (index_name)

In Index

Indicates whether a column belongs to the index

Sort

Sort sequence

Name

Column name

Data Type

Column data type

You cannot use LOB columns in an index.

Unique

Indicates whether the values for this columns must be unique

UNIQUE Definition (unique_definition)

Disabled

Indicates whether the index is deactivated

Usage Count

Indicates how often the system has used the index

Bad

Indicates whether the index is damaged

Since indexes are derived entirely from data already in the database, you can delete them without data being lost.

Note that, in certain circumstances, the database system may have to update the index corresponding to any data changes, which takes time.

Index States

State

Description

OK

The index is intact.

BAD

The index is damaged.

The index’s B* tree is damaged and the database system can no longer use it. The database system ignores damaged indexes. These cannot cause errors, but do impair performance.

MISSING

The index is available in the database catalog, but is missing in the internal file directory.

You can recreate damaged indexes using Database Studio. If you set the UseAutomaticBadIndexRecreation special database parameter to YES, then the database system automatically recreates damaged indexes.

You can use the EXPLAIN statement to check whether an index is really used when the database system searches for data records.

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