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-From: Brook Milligan
- Momjian on Sat, 18 Dec 1999 15:26:15 -0500 (EST))
-Subject: Re: [HACKERS] Re: [PATCHES] Lock
-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
-
-************
-
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- Sat, 18 Dec 1999 21:12:09 -0800 (PST)
-Date: Sat, 18 Dec 1999 21:12:09 -0800 (PST)
-From: Alfred Perlstein
-Subject: Re: [PATCHES] Lock
-MIME-Version: 1.0
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-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.git / commitdiff