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resolving locking conflicts

From: "Burgess, Trevor - HMS" <Trevor(dot)Burgess(at)haysdx(dot)co(dot)uk>
To: "'pgsql-interfaces(at)postgresql(dot)org'" <pgsql-interfaces(at)postgresql(dot)org>
Subject: resolving locking conflicts
Date: 1999-07-14 12:04:06
Message-ID: E15EABE86791D011A1330000F800A1236B6F66@UKWFMKEXCH (view raw or flat)
Thread:
Lists: pgsql-interfaces
I am using Postgres 6.5 on Linux through the libpq interface

I have a situation where an existing record can be opened by one user,
updated or rolled back. While the first user has the record I need to make
sure no other user can open it. I use

SELECT .... FOR UPDATE

This works but the second process blocks (looking to the user like a hung
program), until the first process releases the record. Since updating a
record can take some time the blocking could be there for some time.

I would like the second SELECT .... FOR UPDATE to fail with an error I can
catch telling me of a conflict (and hopefully which process has the lock),
allowing me to backout gracefully with a consolatory message to the user. I
would like normal SELECTs (without FOR UPDATE) to work an normal.

I think that this is an option in pg_options but I'm not sure. I have looked
in the source (lock.c and lmgr.c) to try and figure out what to do. I found
something called "user locks" which looks promising but I'm still not sure.

Help !

Trevor
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