Index Range Scan

An index range scan is an ordered scan of an index in which one or more leading columns of an index are specified in conditions, and 0, 1, or more values are possible for an index key.

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A condition specifies a combination of one or more expressions and logical (Boolean) operators. It returns a value of TRUE, FALSE, or UNKNOWN.

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The database commonly uses an index range scan to access selective data. The selectivity is the percentage of rows in the table that the query selects, with 0 meaning no rows and 1 meaning all rows. Selectivity is tied to a query predicate, such as WHERE last_name LIKE 'A%', or a combination of predicates. A predicate becomes more selective as the value approaches 0 and less selective (or more unselective) as the value approaches 1.

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For example, a user queries employees whose last names begin with A. Assume that the last_name column is indexed, with entries as follows:

CopyAbel,rowid
Ande,rowid
Atkinson,rowid
Austin,rowid
Austin,rowid
Baer,rowid
.
.
.

The database could use a range scan because the last_name column is specified in the predicate and multiples rowids are possible for each index key. For example, two employees are named Austin, so two rowids are associated with the key Austin.

An index range scan can be bounded on both sides, as in a query for departments with IDs between 10 and 40, or bounded on only one side, as in a query for IDs over 40. To scan the index, the database moves backward or forward through the leaf blocks. For example, a scan for IDs between 10 and 40 locates the first index leaf block that contains the lowest key value that is 10 or greater. The scan then proceeds horizontally through the linked list of leaf nodes until it locates a value greater than 40.

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