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The UNIQUE or DISTINCT keyword
in a CREATE INDEX or insert statement
specifies that a secondary access method cannot insert multiple
occurrences of a key value. The UNIQUE or DISTINCT keyword
in a SELECT statement specifies that the
access method must return only one occurrence of a key value.
To provide support for unique keys
- Program
the am_insert purpose function to scan an
index before it inserts each new entry and raise an exception for
a key value that the index already contains.
- Program
the am_getnext to return only one occurrence
of a key.
- Set
the am_unique purpose flag, as described in Setting Purpose Functions, Flags, and Values.
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