Index Unique Scan

In contrast to an index range scan, an index unique scan must have either 0 or 1 rowid associated with an index key.

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The database performs a unique scan when a predicate references all of the columns in the key of a UNIQUE index using an equality operator. An index unique scan stops processing as soon as it finds the first record because no second record is possible.

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As an illustration, suppose that a user runs the following query:

CopySELECT *
FROM   employees
WHERE  employee_id = 5;

Assume that the employee_id column is the primary key and is indexed with entries as follows:

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Copy1,rowid
2,rowid
4,rowid
5,rowid
6,rowid
.
.
.

In this case, the database can use an index unique scan to locate the rowid for the employee whose ID is 5.

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