Yeah, I've read that one a few times. It doesn't really explain a lot of
detail. It only says
"Note, however, that the write transaction does need to use locks"
I was just having a dumb moment... I mean what is the point of FOR
UPDATE if it doesn't get a lock?
The H2 Documentation says
"When using MVCC in this database, delete, insert and update operations
will only issue a shared lock on the table. An exclusive lock is still
used when adding or removing columns, when dropping the table, and when
using SELECT ... FOR UPDATE"
So just to clarify my understanding. I can update / insert / delete in
MVCC without getting exclusive locks, so really it doesn't block other
writers, and still ensures transaction isolation. ?
If I do FOR UPDATE, it gets an exclusive lock (Table level?), so this
would block readers and writers. But I'm having trouble thinking of a
use case for this.
Thanks, Ryan
On 27/02/2013 4:49 PM, Noel Grandin wrote:
On 2013-02-27 10:19, Ryan How wrote:
This doesn't really help, but I thought the idea of MVCC is to be an
optimistic lock?. So by definition it shouldn't be able to timeout as
it doesn't even need to get a "lock" ? Or am I grossly
misunderstanding something here? Does "FOR UPDATE" still lock a table
for other "writers", it just doesn't lock it for other "readers" ?
Try reading about MVCC first.
http://en.wikipedia.org/wiki/Multiversion_concurrency_control
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