Re: EXPLAIN PLAN madness!

In a sudden spasm of awareness,
Ed Stevens doodled thusly:

>OUCH!!! I didn't know that. That puts a real crimp in things. I am looking at
>some additional indexes, was planning on using the plan cost as an assesment of
>their impact . . . . I know that plan cost is a bit of a synthetic number, but
>if you can't use relative values (plan A with a cost of 250000 vs plan B with a
>cost of 250) to select the best course of action . . . . where does that leave
>us?

Woah, calm down! :-)

You can, in the same database, with the same settings, using the same data, rely on the costs put out by EXPLAIN PLAN as a _relative_ indicator of how fast things will go.

That allows you to do what-if tests for a specific SQL to see what is the best solution, without having to retrieve all data and timing that.

What you shouldn't do is pick up a cost in one database and compare it to a cost obtained in another database and establish any relationship between the two based on their absolute value. That simply doesn't work. At least not reliably.

Cheers
Nuno Souto
nsouto_at_optushome.com.au.nospam Received on Sat Nov 17 2001 - 19:27:53 CST