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Re: unique index on fields with possible null values

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Dmitry Ruban <dmitry(at)ruban(dot)biz>
Cc: pgsql-sql(at)postgresql(dot)org
Subject: Re: unique index on fields with possible null values
Date: 2007-07-27 02:49:31
Message-ID: 6521.1185504571@sss.pgh.pa.us (view raw, whole thread or download thread mbox)
Thread:
Lists: pgsql-sql
Dmitry Ruban <dmitry(at)ruban(dot)biz> writes:
> I'm trying to find a solution for unique index on fields with possible
> null values.

You appear to be hoping that a unique index would constrain a column to
contain at most one null entry.  It doesn't work like that, and I
strongly urge you to reconsider what you're using null for.  I think you
are in for a world of hurt, well beyond this one particular point,
because the SQL spec's semantics for null are not designed around the
idea that it represents a single distinguishable value.  What it
represents is "unknown".

			regards, tom lane

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