Re: double linked list

From: Mikito Harakiri <>
Date: Wed, 5 Feb 2003 18:08:08 -0800
Message-ID: <uxj0a.18$>

"DA Morgan" <> wrote in message
> --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 Thu Feb 06 2003 - 03:08:08 CET

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