Re: can UNIQUEness of TEXT datatype really be guaranteed?

<fmiddleton(at)verizon(dot)net> writes:
> So I can't help but wonder, can Postgres really guarantee a TEXT field to be UNIQUE... or is declaring a TEXT field UNIQUE something an uninformed, novice user would do?... or is it something indicative of the strength and/or weeknesses that separate the functionality of the two DBMSs.

In PG, it will work as long as no entry is too large to fit into a btree
index entry (from memory, about 2700 bytes after compression, so the
practical limit is probably 4KB or so).

If you think you might have entries exceeding a few KB, you could use
the trick of declaring a unique functional index on a checksum:
create unique index myindex on mytable (md5(fieldname));
This will work as long as you don't get any md5 hash collisions,
which is probably not a problem in practice. It will guarantee
uniqueness in any case; the risk is that you might get false matches
causing rejection of inputs that actually are distinct.

A possibly simpler-to-understand way is to demand uniqueness in the
first couple KB:
create unique index myindex on mytable (substr(fieldname,1,2000));

regards, tom lane