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Home -> Community -> Usenet -> c.d.o.misc -> Re: double linked list
"DA Morgan" <damorgan_at_exesolutions.com> wrote in message
news:3E41B770.940B52B9_at_exesolutions.com...
> --CELKO-- wrote:
>
> > <snipped>columns are indexed, it is reasonably fast. <<
> >
> > >> What do you mean by "direct use"? This is SQL, a user is allowed to
> > inline and nest subqueries. <<
> >
> > But you want to avoid them in favor of simple, "flat" joins whenever
> > possible. They hurt performance and maintainability.
> >
>
> Not in Oracle.
>
> The performance impact of inline views, while there must be some, is
> unmeasurable. And, in fact, I can easily demonstrate cases where
> subqueries are substantially faster than flat joins and do so in the
> EXPLAIN PLAN lesson I teach first quarter students.
I would be interested to see those. Evaluating subquery for each row of the outer query (Tuple Iteration Semantics) is essentially Nested Loops. Unnesting subquery is a transformation that makes the join explicit. Combining all joins together into a flat select-project-join query is beneficial because it opens larger space of join orders (For example, new join orders might be explored, unavailable in the original query, plus other join methods -- Merge Join and Hash Join are available as well). Received on Wed Feb 05 2003 - 20:08:08 CST
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