>>>>> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:
Tom> "Jenny -" <nat_lazy@hotmail.com> writes: >> Iam trying to acquire rowlevel locks in postgresql. I try doing
>>this: 'select * from students where name='Larry' for update; >> But by looking at the holding array of proclock ,
I'venoticed >> that by doing this only AccessShareLock gets acquired which is >> a table level lock.
Tom> Row-level locks are not recorded in proclock --- they are Tom> implemented by marking the individual tuple
on-disk. If we Tom> tried to record them in shared memory, it'd be very easy to Tom> run out of shared memory,
becauseyou could be holding row Tom> locks on a large number of tuples.
Of course, other database systems do this without too much hassle
.. including relying on lock escalation (move up to page/table level
locks) when the number of locks grow too large.
Does pgsql only record X locks on the individual tuples on-disk or
does it do so for S locks as well ?
Not that I dislike the idea - Toby Lehman suggested this in his
Ph.D. thesis in the mid-eighties for main-memory databases (where you
don't take the write penalty).
--
Pip-pip
Sailesh
http://www.cs.berkeley.edu/~sailesh
