Re: Deadlocks? What happened to MVCC?

Josh Berkus wrote:
> Folks:
>
> I'm getting deadlock errors on one of the operations on my web
> application.  It's a function which adds a large number of rows to a
> holding table, then updates that set of rows multiple times in order to
> present scoring information to the user.
>
> However, the function is deadlocking itself once it's in heavy use.  I
> though that MVCC was supposed to supercede deadlocks, unless I
> specifically set locks, which I'm not.
>
> Help, anyone?
   You   must   have  misunderstood  something.  MVCC  does  not   supersede deadlocks, nor can it prevent them. All
MVCC does   is to help "readers" not beeing blocked by "writers" and vice   versa, by avoiding the need for  shared
"read" locks.   But   writers  still  need  locks,  so  if  your  application  does   something like
 
       Xact-1: Starts transaction       Xact-2: Starts transaction       Xact-1: Updates row A       Xact-2: Updates
rowB       Xact-1: Wants to update row B (blocks)       Xact-2: Wants to update row A
 
   then it's a deadlock. A deadlock is a deadlock and remains to   be a deadlock, no matter if you have versioning or
not.


Jan

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