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From: Heikki Linnakangas <[email protected]>
To: Tom Lane <[email protected]>
Cc: Alvaro Herrera <[email protected]>
Cc: Simon Riggs <[email protected]>
Cc: [email protected]
Subject: Re: FOR SHARE vs FOR UPDATE locks
Date: Fri, 01 Dec 2006 16:55:56 +0000
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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Tom Lane wrote:
> Alvaro Herrera <[email protected]> writes:
>> I'm not sure we can use the simple "raise an ERROR" answer though,
>> because for users that would be a regression.
> 
> I've reconsidered the idea of upgrading the outer xact's shared lock to
> exclusive: at first I thought that would be hard to implement correctly,
> but now I realize it's easy.  Just re-use the XID that's in the multixact
> as the one to store as the exclusive locker, instead of storing our
> current subxact XID.  In some cases this will be a subcommitted XID of
> the current subxact or a parent, but the locking semantics are the same,
> and even though we think such an XID is finished everyone else will see
> it as still live so the appearance of its XID in an XMAX field shouldn't
> be an issue.

That way, the lock won't be downgraded back to a shared lock on 
"rollback to savepoint", right? Though it's still better than throwing 
an error, I think.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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