Re: Query Optimiser Question

not "tremendous" overhead, but there is some additional work to compute and compare the relative "cost" of each step of each possible execution plan. the goal would be that the overhead is more than offset by the selection of "optimal" execution plans.

"Paul" <paul_at_yeahright.com> wrote in message news:xncX4.117$5Y.3520_at_news11-gui.server.ntli.net...
> Wow thats some answer you gave me there buddy :) I think I get
 the drift of
> it though i.e. the optimiser does just what its meant to do
 and selects what
> it thinks is the best solution rather than a number of best
 solutions or all
> the solutions. However it would only be situationally the
 best solution as
> the DB environment changes all the time (Yes?). Thus there is
 need for
> recompilation of DBs statistics and then reassessment and
 calculation by the
> optimiser of what it the best solution based on those
 statistics. If this
> is a regular occurence then am i correct in assuming it would
 be a tremdous
> overhead for systems that are very active 24 hours a day 7
 days a week?
> Thanks
> Paul
>
> Jaap W. van Dijk <j.w.vandijk_at_kpn.com> wrote in message
> news:01bfc65a$c4c4a080$2d2b15ac_at_HKTGN0003049994...
> > I think soultion space is a algebraical notion: if you
 regard all the
> > permissible intervals of all the relevant parameters as
 pieces of axes in
 a
> > mutidimensional space, you've got the solution space: the
 space of all
> > possible solutions. If you then define criteria specifying
 how the
> > parameters are allowed to depend on each other, you limit
 yourself to a
> > part of the total solution space. The criteria an optimizer
 uses
 determines
> > how big this reduction of the solution space will be.
> >
> > Jaap.
> >
> >
> > Paul <paul_at_yeahright.com> schreef in artikel
> > <Lk9X4.112$5Y.2956_at_news11-gui.server.ntli.net>...
> > > I have a question sitting here infront of me :
> > >
> > > Describe how cost based query optimisers work and include
 an explanation
 as
> > > to how they reduce the solution space and what aspects are
 compared by
 the
> > > optimiser.
> > >
> > > Its just the solution space i am abit puzzled by. Does
 this refer to
 the
> > > amount of space taken up by solution strategies in the
 DBMS? If so how
 does
> > > taking up large amounts of so called solution space lead
 to problems?
> > >
> > > Thanx
> > > Paul
> > >
> > >
> > >
>
>
>
Received on Sat May 27 2000 - 00:00:00 CDT