# Re: more on delete from join

Date: Wed, 2 Sep 2009 08:19:30 -0700 (PDT)

Message-ID: <acce30bf-51c4-4f60-aff7-6a8afc279431_at_13g2000prl.googlegroups.com>

On Sep 2, 5:31 am, "Walter Mitty" <wami..._at_verizon.net> wrote:

*> "Marshall" <marshall.spi..._at_gmail.com> wrote in message
**>
*

> news:eb8688da-8577-4088-9d0f-b4b46b1cb696_at_p10g2000prm.googlegroups.com...

*> On Sep 1, 8:03 am, Kevin Kirkpatrick <kvnkrkpt..._at_gmail.com> wrote:
**> <qoute>
**> And if it still looks like guessing to you, you still haven't
**> grasped the concept. Is solving systems of _linear_
**> equations "guessing?" No; it is a process that is entirely
**> deterministic and arithmetically sound. The identical
**> qualities hold under the idea of solving systems of
**> relational expressions.
**> </quote>
**>
**> A minor quibble. It is a process whose outcome is entirely deterministic.
**> The process itself may rely on heuristics to determine the course of action
**> needed to solve the system of equations.
*

Sure. But note that an optimizer needs to
1) find the fastest way

2) to determine the query results.

It is heuristic in 1, algorithmic in 2.

Marshall

> When I learned algebra in high school, we learned various methods of

*> solving systems of equations. Only one of them, the determinant method, was
**> guaranteed to work for every solvable system. And the determinant method
**> was a royal pain. So, if you could find one of the easier methods that
**> worked, such as removing common factors, you could solve the system much
**> more easily.
**>
**> You see, a solution strategy really is an imperative: do this, then do
**> that, then do that, and you'll get the solution. The validity of a
**> strategy is a matter of strict logic. The applicability of a given strategy
**> to a given problem might or might not be a matter of strict logic. It
**> certainly wasn't taught that way, when I took algebra.
**>
**> All of this reminds me a little bit of query optimization in a DBMS.
**> Composers of queries are encouraged to think of the query as specifying what
**> is to be found, rather than how to find it. The DBMS, on the other hand,
**> generates an imperative strategy for carrying out the request. The strategy
**> generator portion of an optimizer certainly uses heuristics for generating
**> strategies to be cost evaluated. Hopefully, the strategy generator
**> generates no incorrect strategies, just some costly ones.
*

Received on Wed Sep 02 2009 - 17:19:30 CEST