Re: Doubt about AccessExclusiveLock in ALTER TABLE .. SET ( .. );

On Tue, Apr 7, 2015 at 10:57 PM, Fabrízio de Royes Mello <
fabriziomello(at)gmail(dot)com> wrote:
>
>
> On Tue, Apr 7, 2015 at 10:15 PM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
wrote:
> >
> > Fabrízio de Royes Mello wrote:
> > > On Mon, Apr 6, 2015 at 12:53 AM, Alvaro Herrera <
alvherre(at)2ndquadrant(dot)com>
> > > wrote:
> > > >
> > > > Fabrízio de Royes Mello wrote:
> > > >
> > > > > Ok guys. The attached patch refactor the reloptions adding a new
field
> > > > > "lockmode" in "relopt_gen" struct and a new method to determine
the
> > > > > required lock level from an option list.
> > > > >
> > > > > We need determine the appropriate lock level for each reloption:
> > > >
> > > > I don't think AccessShareLock is appropriate for any option
change. You
> > > > should be using a lock level that's self-conflicting, as a minimum
> > > > requirement, to avoid two processes changing the value concurrently.
> > >
> > > What locklevel do you suggest? Maybe ShareLock?
> >
> > ShareUpdateExclusive probably. ShareLock doesn't conflict with itself.
> >
>
> Ok.
>
>
> > > > (I would probably go as far as ensuring that the lock level
specified in
> > > > the table DoLockModesConflict() with itself in an Assert somewhere.)
> > >
> > > If I understood this is to check if the locklevel of the reloption
list
> > > don't conflict one each other, is it?
> >
> > I mean
> > Assert(DoLockModesConflict(relopt_gen->locklevel,
relopt_gen->locklevel));
> >
>
> Understood... IMHO the better place to perform this assert is in
"initialize_reloptions".
>
> Thoughts?
>

The attached patch:
- introduce new field "lockmode" to "relopt_gen" struct
- initialize lockmode with ShareUpdateExclusiveLock to autovac settings
- add assert to ensuring that the lock level specified don't conflict with
itself
- add function GetRelOptionsLockLevel to determine the required lock mode
from an option list

Regards,

--
Fabrízio de Royes Mello
Consultoria/Coaching PostgreSQL
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