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Managing Constraints That Have Associated Indexes

Managing Constraints That Have Associated Indexes

When you create a UNIQUE or PRIMARY key, Oracle Database checks to see if an existing index can be used to enforce uniqueness for the constraint. If there is no such index, the database creates one.

Minimizing Space and Time Overhead for Indexes Associated with Constraints

When Oracle Database uses a unique index to enforce a constraint, and constraints associated with the unique index are dropped or disabled, the index is dropped. To preserve the statistics associated with the index (because, for example, it would take a long time to re-create it), you can specify the KEEP INDEX clause on the DROP command for the constraint.

While enabled foreign keys reference a PRIMARY or UNIQUE key, you cannot disable or drop the PRIMARY or UNIQUE key constraint or the index.

Note: UNIQUE and PRIMARY keys with deferrable constraints must all use non-unique indexes.

To reuse existing indexes when creating unique and primary key constraints, you can include USING INDEX in the constraint clause. For example:

CREATE TABLE b

(

b1 INTEGER,

b2 INTEGER,

CONSTRAINT unique1 (b1, b2) USING INDEX (CREATE UNIQUE INDEX b_index on b(b1, b2),

CONSTRAINT unique2 (b1, b2) USING INDEX b_index

);

Guidelines for Indexing Foreign Keys

You should almost always index foreign keys. The only exception is when the matching unique or primary key is never updated or deleted.

See Also: Oracle Database Concepts for information on locking mechanisms involving indexes and keys

3-14 Oracle Database Application Developer's Guide - Fundamentals

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