Erik Jones wrote:
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.
It also won't work because I need to change AND read the row. If I only
do the update, I don't know what was updated. I still need to return
the tcq_id to my application.
Maybe the update could look like this:
UPDATE queue SET
status = in_new_status,
ts_start = NOW(),
ts_end = NULL,
hostname = COALESCE(in_hostname, hostname),
WHERE status = in_status;
But there I don't have the LIMIT 1, and I also don't know which rows got
updated. I supposed there might be some magic to find the OID of the
affected rows, but I don't know how what would be done.
I need logic like "atomic test and set" or pop 1 item off the queue
atomically and tell me what that item was.
In my situation, there are a dozen or so machines polling this queue
periodically looking for work to do. As more polling is occurring, the
locks seem to be taking longer so I was worried table-level locks might
be occurring.
-- Dante
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