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

On Mon, Aug 3, 2015 at 2:15 AM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
wrote:
>
> On Sat, Aug 1, 2015 at 9:20 PM, Andres Freund wrote:
> > On August 1, 2015 2:17:24 PM GMT+02:00, Michael Paquier wrote:
> >>> For instance, if you told me to choose between ShareLock and
> >>> ShareUpdateExclusiveLock I wouldn't know which one is strongest. I
> >>> don't it's sensible to have the "lock mode compare" primitive
> >>honestly.
> >>> I don't have any great ideas to offer ATM sadly.
> >>
> >>Yes, the thing is that lowering the lock levels is good for
> >>concurrency, but the non-monotony of the lock levels makes it
> >>impossible to choose an intermediate state correctly.
> >
> > How about simply acquiring all the locks individually of they're
different types? These few acquisitions won't matter.
>
> As long as this only applies on master, this may be fine... We could
> basically pass a LOCKMASK to the multiple layers of tablecmds.c
> instead of LOCKMODE to track all the locks that need to be taken, and
> all the relations open during operations. Would it be worth having
> some routines like relation_multi_[open|close]() as well? Like that:
> Relation[] relation_multi_open(relation Oid, LOCKMASK):
> void relation_multi_close(Relation[]);
>
> If we do something, we may consider patching as well 9.5, it seems to
> me that tablecmds.c is broken by assuming that lock hierarchy is
> monotone in AlterTableGetLockLevel().
>

Hey guys,

Are you sure we need to do all this changes just to check the highest
locklevel based on the reloptions? Or I misunderstood the whole thing?

Maybe we just check the DoLockModeConflict in the new method
GetReloptionsLockLevel and if true then fallback to AccessExclusiveLock (as
Michael suggested in a previous email).

Regards,

*** This work is funded by Zenvia Mobile Results (http://www.zenvia.com.br)

--
Fabrízio de Royes Mello
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