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+From owner-pgsql-hackers@hub.org Sat Dec 18 17:22:09 1999
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+Date: Sat, 18 Dec 1999 16:14:50 -0700 (MST)
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+From: Brook Milligan <brook@biology.nmsu.edu>
+To: pgman@candle.pha.pa.us
+CC: peter_e@gmx.net, pgsql-hackers@postgreSQL.org
+In-reply-to: <199912182026.PAA05926@candle.pha.pa.us> (message from Bruce
+ Momjian on Sat, 18 Dec 1999 15:26:15 -0500 (EST))
+Subject: Re: [HACKERS] Re: [PATCHES] Lock
+References: <199912182026.PAA05926@candle.pha.pa.us>
+Sender: owner-pgsql-hackers@postgreSQL.org
+Status: OR
+
+ > > * Allow LOCK TABLE tab1, tab2, tab3 so all tables locked in unison
+
+ Let me add to this. One problem is that my description would sometimes
+ lock the tables in different orders, and that is a recipe for deadlock.
+
+ If you have to release earlier locks to wait on a later lock, once you
+ get the later lock, you must release it and then start from the
+ beginning, locking them in order again. If you don't, the system could
+ report a deadlock at random times, which would be very bad.
+
+I'll add something, too. :) I think this derived from a suggestion I
+made long ago. My idea was that when multiple tables need locking, a
+deadlock can occur in the process of doing them one at a time. My
+suggested solution was based on an analogy with the way ethernet
+packets work.
+
+- go through the list locking tables along the way.
+
+- if a lock cannot be obtained within some time, release some (all?) locks,
+ and try again after some random time.
+
+- keep trying (and releasing as needed) until some other timeout
+ passes, and then punt.
+
+My thought was that if colliding locks are occuring, some sequence of
+relinquishing locks (not necessarily all of them with each trial),
+waiting, and reasserting them should work around the collisions.
+Introducing random components to this might reduce the overall waiting
+time, but I suppose a careful analysis of this needs to be done.
+Perhaps just releasing all of the locks, waiting a random time, and
+trying again is enough.
+
+Somehow there has to be a mechanism for atomically asserting locks on
+more than one table.
+
+Cheers,
+Brook
+
+************
+
+From owner-pgsql-patches@hub.org Sat Dec 18 22:51:06 1999
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+Date: Sat, 18 Dec 1999 21:12:09 -0800 (PST)
+From: Alfred Perlstein <bright@wintelcom.net>
+To: Bruce Momjian <pgman@candle.pha.pa.us>
+cc: Peter Eisentraut <peter_e@gmx.net>, patches@postgreSQL.org
+Subject: Re: [PATCHES] Lock
+In-Reply-To: <199912181828.NAA01486@candle.pha.pa.us>
+Message-ID: <Pine.BSF.4.21.9912182107170.12109-100000@fw.wintelcom.net>
+MIME-Version: 1.0
+Content-Type: TEXT/PLAIN; charset=US-ASCII
+Sender: owner-pgsql-patches@postgreSQL.org
+Precedence: bulk
+Status: OR
+
+On Sat, 18 Dec 1999, Bruce Momjian wrote:
+
+> [Charset ISO-8859-1 unsupported, filtering to ASCII...]
+> > I was looking at this
+> >
+> > * Allow LOCK TABLE tab1, tab2, tab3 so all tables locked in unison
+> >
+> > but I'm not sure if my solution is really what was wanted, because it
+> > doesn't actually guarantee an all-or-nothing lock, it just locks each
+> > table in order. Thus it's more like a syntax simplification and reduces
+> > overhead.
+> >
+>
+> It took a few minutes, but I remember the use for this. If you are
+> going to hang waiting to lock tab3, you don't want to lock tab1 and tab2
+> while you are waiting for tab3 lock. The user wanted all tables to lock
+> in one operation without holding locks while waiting to complete all
+> locking.
+>
+> Can you do the locks, and if one fails, not hang, but unlock the
+> previous tables, go lock/hang on the failure, and go back and lock the
+> others? Seems it would have to be some kind of lock/fail/unlock/wait
+> loop.
+>
+> Does this make sense? It did to me.
+
+Guys, have a look at:
+
+http://www.freebsd.org/~terry/iml.txt
+http://jazz.external.hp.com/training/sqltables/c5s17.html
+
+It's a way to do locking with deadlock detection, and without loosing
+your place in line for locks, very nifty imo.
+
+-Alfred
+
+
+************
+
+
projects / postgresql / commitdiff