The following bug has been logged online:

Bug reference: 1748
Logged by: Greg Steffensen
Email address: greg.steffensen (AT) gmail (DOT) com
PostgreSQL version: 8.02
Operating system: Gentoo Linux
Description: Unique contraints cannot be added to long text fields
Details:

I'm storing SVG files, which can be many kilobytes long, in a text field.
When I try to add a unique contraint to that field, or try to add a row to a
table that already has had the unique contraint applied, I get errors like
the following:

ERROR: index row requires 15528 bytes, maximum size is 8191

Evidently, the unique constraint creates an index, but the index can't
handle large field sizes.

---------------------------(end of broadcast)---------------------------
TIP 3: if posting/reading through Usenet, please send an appropriate
subscribe-nomail command to majordomo (AT) postgresql (DOT) org so that your
message can get through to the mailing list cleanly

On Sat, Jul 02, 2005 at 05:20:29PM +0100, Greg Steffensen wrote:

Yes, that's a known limitation -- you cannot store arbitrarily large
fields in an index. Even if it was possible I don't think it'd be a
good idea from an efficiency POV. You may want to create an index in a
digested version of the file instead (SHA-1, say, or MD5).

--
Alvaro Herrera (<alvherre[a]surnet.cl>)
"El conflicto es el camino real hacia la unión"

---------------------------(end of broadcast)---------------------------
TIP 8: explain analyze is your friend