Specify ENABLE if you want the constraint to be applied to the data
in the table.

If you enable a unique or primary key constraint, and if no index exists on the key,
then Oracle Database creates a unique index. Unless you specify KEEP INDEX when
subsequently disabling the constraint, this index is dropped and the database rebuilds
the index every time the constraint is reenabled.

You can also avoid rebuilding the index and eliminate redundant indexes by creating
new primary key and unique constraints initially disabled. Then create (or use
existing) nonunique indexes to enforce the constraint. Oracle does not drop a
nonunique index when the constraint is disabled, so subsequent ENABLE operations
are facilitated.

If you specify neither VALIDATE nor NOVALIDATE, the default is VALIDATE.

If you change the state of any single constraint from ENABLE NOVALIDATE to ENABLE
VALIDATE, the operation can be performed in parallel, and does not block reads,
writes, or other DDL operations.

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ENABLE Clause

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