Re: Unique Index

Greg Stark <gsstark(at)mit(dot)edu> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> Not for UNIQUE constraints. SQL92 section 4.10 "Integrity constraints":
>>
>> A unique constraint is satisfied if and only if no two rows in
>> a table have the same non-null values in the unique columns.

> That's ambiguous. Does it mean no two rows have all non-null columns that are
> all identical? Or does it mean no two rows have columns that excluding any
> null columns are identical.

OK, try the more formal definition in 8.9 <unique predicate>

2) If there are no two rows in T such that the value of each column
in one row is non-null and is equal to the value of the cor-
responding column in the other row according to Subclause 8.2,
"<comparison predicate>", then the result of the <unique predi-
cate> is true; otherwise, the result of the <unique predicate>
is false.

(11.7 defines the UNIQUE constraint in terms of the unique predicate)

regards, tom lane