AW: AW: AW: AW: relation ### modified while in use

> > You were talking about the "select only" case (and no for update eighter). 
> > I think that select statements need a shared lock for the duration of their 
> > execution only.
> 
> You seem to think that locks on individual tuples conflict with
> table-wide locks.

Yes, very much so. Any other way would be subject to the same quirks 
you would like to avoid, no ?

>  AFAIK that's not true.

well, imho room for improvement.

> The only way to prevent
> another xact from gaining AccessExclusiveLock on a table is to be
> holding some lock *on the table*.

Yes, and holding a row exclusive lock must imho at least grab a shared
table lock (to avoid several problems, like missing an index update,
inserting a null into a newly added not null column ...).
Alternately the table exclusive lock could honour row locks 
(probably not possible, since we don't track those do we ?).

> As for your claim that read-only xacts don't need to worry about
> preventing schema updates, what of adding/deleting ON SELECT rules?

Well, depends on what that rule does, you mean a new rule ?
Ad hoc I don't see a problem based on the idea that all modification gets 
appropriate locks.

Andreas