Re: [SQL] Good Optimization

Tom Lane wrote:

> secret <secret@kearneydev.com> writes:
> >     There is a simple way to optimize SQL queries involving joins to
> > PostgreSQL that I think should be handled by Postgre?  If one is joining
> > a tables a,b on attribute "x" and if one has something like x=3 then it
> > helps A LOT to say: a.x=3 and b.x=3 in addition to saying a.x=b.x ...
> > The example below shoulds the radical speed gain of doing this, and I
> > think it isn't something real obvious to most people...
>
> How much *actual* speedup is there?  I don't trust the optimizer's
> numbers as anything more than relative measures ;-)
>
> I'm a bit surprised that you are getting a nested-loop plan and not
> a merge or hash join.  With a merge join, at least, there ought not be
> a large difference from providing the additional qual clause (I think).
> What Postgres version are you using?
>
>                         regards, tom lane
   The actual performance difference is HUGE.  Hours vs minutes or Minutes vs
Seconds...

David Secret
MIS Director
Kearney Development Co., Inc.