# 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 - 10:19:30 CDT