BUG #3965: UNIQUE constraint fails on long column values

The following bug has been logged online:

Bug reference: 3965
Logged by: Juho Saarikko
Email address: juhos(at)mbnet(dot)fi
PostgreSQL version: 8.3RC2
Operating system: Linux
Description: UNIQUE constraint fails on long column values
Details:

It is impossible to add an UNIQUE constraint which includes columns with
long values. The reason seems to be that UNIQUE is implemented using b-tree
index, which cannot handle values longer than 8191 bytes.

While I didn't test, I'd imagine that this would also mean that any attempt
to insert such values to an already unique column would fail.

It is propably impossible to fix this in a simple way, since it is an
inherent result of the underlying storage specification rather than a mere
programming error, so the documentation needs to be updated to warn about
this.

I suggest implementing unique hash indexes and automatically creating one
(and turning the b-tree index into a non-unique one) when a large value is
inserted to fix this. Alternatively, fix b-trees so they can handle large
values; however, a hash index should be far more efficient for this specific
case, since the size of a hash is independent of pre-hash data size.

Exact error message:
******************
kuvat=# alter table pictures ADD constraint pic_unique unique (safe);
NOTICE: 00000: ALTER TABLE / ADD UNIQUE will create implicit index
"pic_unique" for table "pictures"
LOCATION: DefineIndex, indexcmds.c:434
ERROR: 54000: index row requires 47148 bytes, maximum size is 8191
LOCATION: index_form_tuple, indextuple.c:170