Re: Sql to Relational Algebra

"Bob Badour" <bbadour_at_golden.net> writes:

>The latter advantage is particularly important for usability. Years ago,
>Fabian Pascal did a comparison of different SQL dbms's ability to provide
>good, consistent performance for logically equivalent queries using
>different redundant elements of SQL. If I recall correctly, the dbms that
>could achieve the fastest performance would also deliver the worst
>performance depending on how the user expressed the query. Ingres delivered
>the most consistent performance because, at the time, it supported SQL by
>translating it to QUEL.

The pros and cons of SQL vs. quel are strictly in terms of human factors of expressing queries. Many people preferred QUEL because it was just syntactic sugar for tuple relational calculus, a well defined paradigm. SQL is a kidn of hybrid of different paradigms, and as such can be less intuitive or semantically clear without looking at the actual language reference to understand the behavior of certain constructs. At one time, Ingres offered more consistent performance, independent of how the user expressed a query, because it had a cost-based optimizer that chose a plan with estimated cost that was optimal in the set of all plans considered by the optimizer (which is a strict subset of the set of all possible plans). Most notably Oracle for many years had only heuristic based optimization and the performance of SQL queries was dependent on the order in which relations were listed in the FROM clause, which dictated join orders in the query plan.

Joseph Received on Sat Sep 20 2003 - 21:54:32 CEST