>>>>> "Tom" == Tom Lane <[EMAIL PROTECTED]> writes:
Tom> "Jenny -" <[EMAIL PROTECTED]> 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've noticed
>> 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, because you 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
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TIP 4: Don't 'kill -9' the postmaster
