It is the part where you can read : "was waiting for a 'next key' lock" that is bugging me.Īm I correct if I understand this as : "I am waiting to get a lock on the next value I need to update" ? Or is there something else to understand?Īlso, trying to minimize the occurrence of this deadlock, would it be a good idea, since I need to update many records, to acquire a table lock before updating my records. Table in database 'tom_data' but process (Familyid 0, Spid 4651) already held a Waiting for a 'next key' lock on row 0 page 12282632 of the 'tom_req_assign' The following has been retrieve from the log :ĭeadlock Id 1: Process (Familyid 0, Spid 4093) was For more information about handling deadlock exceptions, see X++ standards: try/catch Statements. You can then test for a deadlock exception and retry the operation.
The deadlock is a result between an UPDATE WHERE (Process 1) and a SELECT (Process 2). Deadlocks cannot always be avoided so be sure to put database transaction code within a try/catch block. I'll be explaining my proposal in a forthcoming article.I'm having a deadlock that comes along every once in a while, not a big issue but there is something I try to figure out and I am not quite sure what to make of it. Levers within the lock body are lifted by a key which in turn move a deadbolt out of the lock body and into a keep which is situated within the door frame. If you've been affected by this problem and would like to discuss a solution, please let me know in a comment below. The Modern Deadlock The principle of the deadlock design is still to this day the same as the early deadlocks produced by Chubb.
The pgsql-hackers discussion contained some very useful ideas on how to attack this problem, which is what I intend to do. And the reason it's blocked is pretty simple: session 1 is holding a shared lock on the row in table foo, and the UPDATE in session 2 wants to acquire an exclusive lock to be able to modify it. Consider this simple example:ĬREATE TABLE foo (a int PRIMARY KEY, b text) ĬREATE TABLE bar (a int NOT NULL REFERENCES foo) Put simply, certain operations are blocked when there is no need for it. Why, he asked, wasn't the process getting blocked the first time around? His initial explanation was incorrect, but the underlying reason for his problem was that that transaction was getting a shared lock on the referenced row (the one on table A), which it really didn't need except to ensure that the row didn't go away - that is, to make sure the foreign key constraint remained satisfied until it could commit. In Joel's example, he was getting an unexpected deadlock when session 2 updated the row the second time.
Joel Jacobson of Glue Finance illustrated it with an example in the post referenced above, which can be seen in action in this screencast.
This had an enormous positive impact to concurrency, so people then began to use foreign keys more extensively.īut when you start raising the load level, at some point another problem becomes apparent: the locks taken are a stronger than strictly necessary, causing pauses and sometimes deadlocks. We partly fixed this by introducing SELECT FOR SHARE in 8.1, which allowed checks to be run concurrently. Consequently, many people used to drop their foreign keys just to get a reasonable concurrency level. This was so strong a lock that it had a severe impact to the performance of applications that expected to concurrently access and modify tables with foreign key relationships. To recapitulate on this problem a bit: in the aboriginal code, foreign key checks obtained FOR UPDATE locks on referenced tuples, meaning that they were exclusively locked for the duration of the transaction doing the check. The most recent detailed discussion about this problem took place on August 2010 on pgsql-hackers. This problem has been known for a very long time, and it affects many users to varying degrees. This causes some operations to block unnecessarily and, perhaps more aggravating, some other operations to deadlock. More specifically, on the problem that when foreign keys are verified, they sometimes obtain locks that are stronger than really needed.
I've been commissioned to work on foreign keys.