Fast Full Index Scan

A fast full index scan is a full index scan in which the database accesses the data in the index itself without accessing the table, and the database reads the index blocks in no particular order.

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Fast full index scans are an alternative to a full table scan when both of the following conditions are met:

• The index must contain all columns needed for the query.
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• A row containing all nulls must not appear in the query result set. For this result to be guaranteed, at least one column in the index must have either:
  • A NOT NULL constraint
  • A predicate applied to the column that prevents nulls from being considered in the query result set

Example 3-2 Fast Full Index Scan

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Assume that an application issues the following query, which does not include an ORDER BY clause:

CopySELECT last_name, salary
FROM   employees;

The last_name column has a not null constraint. If the last name and salary are a composite key in an index, then a fast full index scan can read the index entries to obtain the requested information:

CopyBaida,2900,rowid
Atkinson,2800,rowid
Zlotkey,10500,rowid
Austin,7200,rowid
Baer,10000,rowid
Austin,4800,rowid
.
.
.

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