Re: Reduce lock levels for ADD and DROP COLUMN

On Mon, 2010-12-27 at 10:41 -0500, Tom Lane wrote:
> Robert Haas  writes:
> > On Mon, Dec 27, 2010 at 6:42 AM, Simon Riggs  wrote:
> >> Idea is to reduce lock level of ADD/DROP COLUMN from AccessExclusiveLock
> >> down to ShareRowExclusiveLock.
> >> 
> >> To make it work, we need to recognise that we are adding a column
> >> without rewriting the table.
> 
> > Can you elaborate on why you think that's the right test?  It seems to
> > me there could be code out there that assumes that the tuple
> > descriptor won't change under it while it holds an AccessShareLock.
> 
> s/could/definitely is/
> 
> I think this is guaranteed to break stuff; to the point that I'm
> not even going to review the proposal in any detail.

Our emails crossed.

Do you disagree with the ADD or the DROP, or both?

What "stuff" will break, in your opinion? I'm not asking you to do the
research, but a few curveballs would be enough to end this quickly, and
leave a good record for the archives.

-- Simon Riggs           http://www.2ndQuadrant.com/books/PostgreSQL Development, 24x7 Support, Training and Services