Thread: tablelevel and rowlevel locks
I am working on a project that involves displaying locking information about each lock taken, whether it be a row level or table leve llock. When dealing with struct LOCK (src/include/storage) i have noticed that postgreSQL creates a single LOCK struct for each table in the db. Like if i acquire 2 seperate row level locks on 2 seperate rows, both these locks are represented in the same struct LOCK datastructure . And if i acquire a table level lock on this table, then all 3 locks information( 2 row level locks and 1 table level lock) is recorded in the same LOCK datastructure. I would think that there should be 3 different LOCK datastructres created in postgreSQL for the above example. IS there are reason why only one LOCK datastructures is created in such a case? for the purposes of my project i need to seperate out these 3 locks and present them as 3 distinct locked objects. i was using the LockData(defined in src/include/storage/lock.h ) datastructures for this, but if all 3 locks are put into the same LOCK struct even in LOCKDATA then can someone help me as to how else i can accomplish what i intent on doing? thank you very much Jenny _________________________________________________________________ Compare Cable, DSL or Satellite plans: As low as $29.95. https://broadband.msn.com
On Thu, Sep 04, 2003 at 08:56:31AM -0700, Jenny - wrote: > I am working on a project that involves displaying locking information > about each lock taken, whether it be a row level or table leve llock. > When dealing with struct LOCK (src/include/storage) i have noticed that > postgreSQL creates a single LOCK struct for each table in the db. Like if > i acquire 2 seperate row level locks on 2 seperate rows, both these locks > are represented in the same struct LOCK datastructure. I think the locks would actually by represented by PROCLOCK structures. The LOCK structures are for lockable objects, not for actual locks. -- Alvaro Herrera (<alvherre[@]dcc.uchile.cl>) "Vivir y dejar de vivir son soluciones imaginarias. La existencia est� en otra parte" (Andre Breton)
>On Thu, Sep 04, 2003 at 08:56:31AM -0700, Jenny - wrote: > > I am working on a project that involves displaying locking information > > about each lock taken, whether it be a row level or table leve llock. > > When dealing with struct LOCK (src/include/storage) i have noticed that > > postgreSQL creates a single LOCK struct for each table in the db. Like >if > > i acquire 2 seperate row level locks on 2 seperate rows, both these >locks > > are represented in the same struct LOCK datastructure. > >I think the locks would actually by represented by PROCLOCK structures. >The LOCK structures are for lockable objects, not for actual locks. > Well,from what i understand, PROCLOCK stores the TransactionID and the LOCK its holding lock on ,so how would PROCLOCK be holding the 'actual' lock as apposed to the lockable objects? thanks! _________________________________________________________________ Use custom emotions -- try MSN Messenger 6.0! http://www.msnmessenger-download.com/tracking/reach_emoticon
On Thu, Sep 04, 2003 at 11:21:05AM -0700, Jenny - wrote: > >I think the locks would actually by represented by PROCLOCK structures. > >The LOCK structures are for lockable objects, not for actual locks. > > Well,from what i understand, PROCLOCK stores the TransactionID and the LOCK > its holding lock on ,so how would PROCLOCK be holding the 'actual' lock as > apposed to the lockable objects? Huh... look at http://developer.postgresql.org/pdf/internalpics.pdf pages 61 and 63. Maybe it's clearer than whatever I can say. Note that "HOLDER" has been renamed to "PROCLOCK". Anyway, I think the LOCK structure represents something that can be locked. The PROCLOCK struct represents that some process is holding a lock on said object. That may be the reason why you are seeing that a lock is held by more than one process at the same time (while in fact some of them are probably _waiting_ for the lock). -- Alvaro Herrera (<alvherre[@]dcc.uchile.cl>) "I dream about dreams about dreams", sang the nightingale under the pale moon (Sandman)
[ pgsql-general removed from cc list, as this is quite inappropriate
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"Jenny -" <nat_lazy@hotmail.com> writes:
> I am working on a project that involves displaying locking information about
> each lock taken, whether it be a row level or table leve llock.
> When dealing with struct LOCK (src/include/storage) i have noticed that
> postgreSQL creates a single LOCK struct for each table in the db. Like if
> i acquire 2 seperate row level locks on 2 seperate rows, both these locks
> are represented in the same struct LOCK datastructure .
As has been pointed out to you several times already, the LOCK
structures aren't used for row-level locks. The objects that you are
looking at represent table-level locks. For example, afterBEGIN;SELECT * FROM foo WHERE id IN (1,2) FOR UPDATE;
there will be a table-level AccessShareLock on "foo" that was acquired
(and not released) by the SELECT statement as a whole. If there
actually were rows matching the WHERE clause, the locks on them are
represented by modifying their tuple headers on-disk. There is nothing
about individual rows in the LOCK table.
Now, if you have modified the code with the intention of creating LOCK
entries for row-level locks, then all I can say is you didn't do it
right. The LockTag created to represent a row-level lock should be
distinct from a table-level LockTag (or a page-level LockTag for that
matter). If it is, the hash table will definitely store it as a
separate object.
regards, tom lane
Alvaro Herrera Munoz <alvherre@dcc.uchile.cl> writes:
> Anyway, I think the LOCK structure represents something that can be locked.
Right.
> The PROCLOCK struct represents that some process is holding a lock on said
> object.
IIRC, a PROCLOCK is created as soon as some backend tries to lock some
lockable object. So the PROCLOCK may only indicate that that backend is
waiting for a lock on that object, not that it already has one.
> That may be the reason why you are seeing that a lock is held by more
> than one process at the same time (while in fact some of them are probably
> _waiting_ for the lock).
Also keep in mind that we use a lot of sharable lock modes --- so it's
entirely likely that multiple processes actually do have locks on the
same object. That's not wrong if their lock modes don't conflict.
regards, tom lane