From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Lincoln Yeoh <lyeoh(at)pop(dot)jaring(dot)my> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Re: offset and limit in update and subselect |
Date: | 2001-02-25 21:58:04 |
Message-ID: | 21151.983138284@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Lincoln Yeoh <lyeoh(at)pop(dot)jaring(dot)my> writes:
> Would it then be fine to use update ... limit in the following scenario?
> I have a todo queue:
> create table todo ( task text, pid int default 0);
> The tasks are inserted into the todo table.
> Then the various worker processes do the following update to grab tasks
> without duplication.
> update todo set pid=$mypid where pid=0 limit 1;
There's no LIMIT clause in UPDATE. You could do something like
BEGIN
SELECT taskid FROM todo WHERE pid = 0 FOR UPDATE LIMIT 1;
UPDATE todo SET pid = $mypid WHERE taskid = $selectedid;
COMMIT
(assuming taskid is unique; you could use the OID if you have no
application-defined ID).
> What would the performance impact of "order by" be in a LIMIT X case? Would
> it require a full table scan?
Yes, unless there's an index on the order-by item. The above example
should be fairly efficient if both pid and taskid are indexed.
Hmm ... trying this out just now, I realize that 7.1 effectively does
the LIMIT before the FOR UPDATE, which is not the way 7.0 behaved.
Ugh. Too late to fix it for 7.1, but I guess FOR UPDATE marking ought
to become a plan node just like LIMIT did.
regards, tom lane
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