On 29/06/10 04:48, Tom Lane wrote:
> "Ross J. Reedstrom"<reedstrm@rice.edu> writes:
>
>> Hmm, I'm suddenly struck by the idea of having a max_cost parameter,
>> that refuses to run (or delays?) queries that have "too high" a cost.
>>
> That's been suggested before, and shot down on the grounds that the
> planner's cost estimates are not trustworthy enough to rely on for
> purposes of outright-failing a query. If you didn't want random
> unexpected failures, you'd have to set the limit so much higher than
> your regular queries cost that it'd be pretty much useless.
>
>
I wrote something along the lines of this for Greenplum (is probably
still available in the Bizgres cvs). Yes, cost is not an ideal metric to
use for bounding workload (but was perhaps better than nothing at all in
the case it was intended for).
One difficulty with looking at things from the statement cost point of
view is that all the requisite locks are already taken by the time you
have a plan - so if you delay execution, these are still held, so
deadlock likelihood is increased (unless you release locks for waiters,
and retry for them later - but possibly need to restart executor from
scratch to cope with possible table or schema changes).
> Maybe it'd be all right if it were just used to delay launching the
> query a bit, but I'm not entirely sure I see the point of that.
>
I recall handling this by having a configurable option to let these
queries run if nothing else was. Clearly to have this option on you
would have to be confident that no single query could bring the system down.
Cheers
Mark