Re: [SQL] Good Optimization

Jan Wieck wrote:
>
> >
> > Bruce Momjian wrote:
> > >=20
> > > Added to TODO:
> > >=20
> > > * In WHERE x=3D3 AND x=3Dy, add y=3D3
> >
> > I don't know if I'm way off, but wouldn't removing "x=3Dy" improve
> > performance further?
>
> Maybe in the simple case above. But it will probably change
> the result set if someone issues
>
> SELECT a.x, b.y FROM a, b WHERE a.x >= 10
> AND a.x <= 19
> AND a.x = b.x;
>
> which should then get rewritten into
>
> SELECT a.x, b.y FROM a, b WHERE a.x >= 10
> AND a.x <= 19
> AND a.x = b.x
> AND b.x >= 10
> AND b.x <= 19;
>
> This time the "a.x = b.x" is important!

Ouch... didn't think that far. However, if an cache is implemented
it might be worth the extra overhead to optimize special cases like
the one I commented on. Since most systems use a fairly limited set
of queries (I suppose...) most of the queries will be cached quite
fast.

BTW, I just love the work all of you are doing! Postgres is awsome!

/Kudo