Re: Sql to Relational Algebra

"Danny deSousa" <Danny_deSousa_at_telus.net> wrote in message news:pan.2003.07.20.00.16.00.856143_at_telus.net...
> Hello,
>
> I'm contemplating writing an Sql parser, more so for my own personal
> education, and am wondering if anyone has had any experience that they can
> share with me on a question that I have.
>
> I have good experience with Sql, since I maintain and develop my companys
Sql
> database engine. The product I maintain parses Sql and constructs a parse
tree
> based on the elements of the Sql statement. Would building the tree using
> "Relational Algebra" elements be better instead? The Sql statement would
> be strictly Dml.
>
> Some advantages that I can think of:
>
> 1. Insulates the core logic of the query processor from the query language
> 2. Provides identical semantics to redundant elements of the query
> language

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.

Fabian mentions QUEL's advantages without rehashing his earlier essay at http://www.pgro.uk7.net/x_terminology.htm Received on Sun Jul 20 2003 - 03:44:00 CEST