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Re: Change lock requirements for adding a trigger

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Decibel! <decibel(at)decibel(dot)org>
Cc: PG Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Change lock requirements for adding a trigger
Date: 2008-06-03 22:04:13
Message-ID: 1212530653.4148.31.camel@ebony.site (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, 2008-06-03 at 16:48 -0500, Decibel! wrote:
> On May 30, 2008, at 9:51 AM, Simon Riggs wrote:
> > On Thu, 2008-05-29 at 19:18 -0500, Decibel! wrote:
> >> Is there a reason that we can't add a trigger to a table while a
> >> select is running? This is a serious pain when trying to setup
> >> londiste or slony.
> >
> > This is constrained by locking.
> >
> > There are a subset of DDL commands that might be able to be performed
> > with just an ExclusiveLock or ShareLock rather than an
> > AccessExclusiveLock. Nobody has studied which sub-statements this  
> > might
> > apply to, but its do-able since CREATE INDEX already does this.
> 
> Is there a good way to determine this other than depending on  
> knowledge of the source code?

The source doesn't know yet. So just analysis and thinking.

The mechanism to hold less than an AccessExclusiveLock it doesn't exist
yet, but it never will unless we have a list of the things that might be
performed correctly with a lower level of lock.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support


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