From: Rod Taylor <rbt@rbt.ca>
To: daniel alvarez <d-alvarez@gmx.de>
Cc: pgsql-sql@postgresql.org
Subject: Re: Atomicity of UPDATE, interchanging values in unique column
Date: 08 Mar 2003 16:33:22 -0500
Message-ID: <1047159202.28251.196.camel@jester> (raw)
In-Reply-To: <32207.1047158233@www5.gmx.net>
References: <32207.1047158233@www5.gmx.net>
> UPDATE sometable SET unique_col =
> CASE WHEN unique_col = firstvalue THEN secondvalue
> ELSE firstvalue
> END
> WHERE unique_col = firstvalue
> OR unique_col = secondvalue
(See last comment)
> BEGIN;
> SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
>
> UPDATE sometable SET unique_col = firstvalue WHERE unique_col = secondvalue;
> UPDATE sometable SET unique_col = secondvalue WHERE unique_col = firstvalue;
>
> COMMIT;
This one will always fail unless you DEFER unique constraints --
something we don't support with PostgreSQL, but some others do.
> How can I interchange two values in a unique column? Am I missing something
> really
> obvious (like a swap statement)? Is there any reason besides performance for
> not
> making index accesses fully ACID-compliant? Doesn't MVCC require this
> anyway?
The first is what you want. PostgreSQL needs some work in the
evaluation of unique indexes to properly support it.
Namely, when it sees a conflict when inserting into the index, it needs
to record the fact, and revisit the conflict at the end of the command.
Lots of work...
--
Rod Taylor <rbt@rbt.ca>
PGP Key: http://www.rbt.ca/rbtpub.ascAttachments:
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To: pgsql-sql@postgresql.org
Cc: rbt@rbt.ca, d-alvarez@gmx.de
Subject: Re: Atomicity of UPDATE, interchanging values in unique column
In-Reply-To: <1047159202.28251.196.camel@jester>
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