Re: BUG #3965: UNIQUE constraint fails on long column values

Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
>
>> Juho Saarikko wrote:
>>
>>> 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.
>>>
>
>
>> Works here in 8.3:
>>
>
>
>>     test=> create table test (x text unique);
>>     NOTICE:  CREATE TABLE / UNIQUE will create implicit index "test_x_key" for table "test"
>>     CREATE TABLE
>>     test=> insert into test values (repeat('a', 50000));
>>     INSERT 0 1
>>
>
> That test only works because it's eminently compressible.
>
>
> The short answer to this bug report is that we're not very concerned
> about fixing this because there is seldom a good reason to have an
> index (unique or not) on fields that can get so wide.  As was already
> noted, if you do need a uniqueness check you can easily make a 99.9999%
> solution by indexing the md5 hash (or some similar digest) of the
> column.  It doesn't really seem worthwhile to expend development work
> on something that would benefit so few people.
>
>             regards, tom lane
>
>
But the documentation needs to be updated to mention this nonetheless.
It is a nasty surprise if it hits unawares.

Besides, it's not such an impossible scenario. I encountered this bug
when making an Usenet image archival system. Since the same images tend
to be reposted a lot, it makes sense to store them only once, and simply
reference the stored image from each context it was posted in. Currently
my program does the uniqueness constraining by itself; I was examining
having the database enforce it when I ran into this issue.

Such applications are not exactly rare: bayimg, img.google.com, etc. and
of course the innumerable Usenet archival sites could all conceivably
want to do something like this. So could any application which monitors
potentially repeating phenomena, for that matter. After all, saving a
single state of the system only once not only reduces the amount of data
stored, but could also help in actual analysis of it, since it becomes
trivial to recognize most and least often recurring states.