Subject: Re: Atomicity of UPDATE, interchanging values in unique column
From: Rod Taylor <rbt@rbt.ca>
To: daniel alvarez <d-alvarez@gmx.de>
Cc: pgsql-sql@postgresql.org
In-Reply-To: <32207.1047158233@www5.gmx.net>
References: <32207.1047158233@www5.gmx.net>
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Date: 08 Mar 2003 16:33:22 -0500

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> UPDATE sometable SET unique_col =3D
>    CASE WHEN unique_col =3D firstvalue THEN secondvalue
>             ELSE  firstvalue
>    END
> WHERE unique_col =3D firstvalue
>       OR unique_col =3D secondvalue

(See last comment)

> BEGIN;
> SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
>=20
> UPDATE sometable SET unique_col =3D firstvalue WHERE unique_col =3D secon=
dvalue;
> UPDATE sometable SET unique_col =3D secondvalue WHERE unique_col =3D firs=
tvalue;
>=20
> 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 somethi=
ng
> 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.=20
Lots of work...

--=20
Rod Taylor <rbt@rbt.ca>

PGP Key: http://www.rbt.ca/rbtpub.asc

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