Re: Clarification, please

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Mladen Gogala <mladen(dot)gogala(at)vmsinfo(dot)com>
Cc: "pgsql-novice(at)postgresql(dot)org >> \"pgsql-novice(at)postgresql(dot)org\"" <pgsql-novice(at)postgresql(dot)org>
Subject: Re: Clarification, please
Date: 2010-12-01 16:59:07
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Mladen Gogala <mladen(dot)gogala(at)vmsinfo(dot)com> writes:
> In Oracle, deferrable primary keys are enforced by non-unique indexes.
> That seems logical,

... maybe to an Oracle guy ...

> When the constraint is deferred in the transaction block, however, it
> tolerates duplicate values until the end of transaction:

Sure. That's exactly per spec: the check is deferred to end of
transaction. If the duplicated index entries are both/all still live
at that time, you get an error.

We do still execute the insertion-time uniqueness check, but instead of
throwing an error on failure, we just queue a trigger event to recheck
that key before commit. If the insertion-time check passes then there's
no need for a recheck later. This is a win because the insertion-time
check is cheap, being integrated into the insertion process itself.

regards, tom lane

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