On Aug 15, 2007, at 2:39 PM, btober@xxxxxxxxxxxxxxxx wrote:
Erik Jones wrote:
On Aug 15, 2007, at 1:09 PM, D. Dante Lorenso wrote:
...to ensure that only one server is processing the queue item,
so inside PGSQL, use SELECT ... FOR UPDATE to lock the row...
When my server is under severe load, however, this function
begins to take a long time to execute and I begin to suspect that
the FOR UPDATE lock might be locking the whole table and not just
the row. How do I know if this is the case, how can I optimize
this procedure, and how should I be doing this differently? ...
Thoughts?
SELECT ... FOR UPDATE should only be locking the rows returned by
your the select statement, in this case the one row. You can
check what locks exist on a table (and their type) with the
pg_locks system view.
Is that correct? Documentation section 12.3.1. Table-Level Locks
states 'The list below shows the available lock modes ...Remember
that all of these lock modes are table-level locks, even if the
name contains the word "row"'.
You will notice that SELECT ... FOR UPDATE is not in that list. It's
covered in the next section on row level locks.
I wonder why bother with the SELECT statement at all. Why not just
go straight to the UPDATE statement with something like
UPDATE queue SET
status = in_new_status,
ts_start = NOW(),
ts_end = NULL,
hostname = COALESCE(in_hostname, hostname)
WHERE tcq_id = (SELECT tcq_id FROM queue q WHERE q.status =
in_status ORDER BY tcq_id ASC LIMIT 1);
He may need to trap an exception for the "not found" case, but
what's the big deal with that?
UPDATE statements acquire a ROW EXCLUSIVE on the table, which
conflicts, among other things, with ROW EXCLUSIVE, so it will block
other UPDATE statements initiated by other transactions.
That won't work because the update won't lock the row until the
select returns. So, if two process execute that at the same time
they will both execute the subquery and return the same result, the
first will update it and the second will then (redundantly) update it.
Erik Jones
Software Developer | Emma®
erik@xxxxxxxxxx
800.595.4401 or 615.292.5888
615.292.0777 (fax)
Emma helps organizations everywhere communicate & market in style.
Visit us online at http://www.myemma.com
---------------------------(end of broadcast)---------------------------
TIP 5: don't forget to increase your free space map settings
