Обсуждение: optimizer question

Hi,

I was just approached with an optimizer question I cannot answer. Does
our optimizer know how expensive different comparisons are? That is can
it do something like:

If there are different comparisons in a where clause check the ints
first, then the strings, then everything with regexp, or like clauses
etc. and finally function calls at last, because in most cases a
function call is the most expensive one. 

Okay, we may argue whether a regexp is more expensive than a function,
but you get the idea. Are we able to get that logic in where clauses
where no index is defined? I just tried a query that given the order in
the where clause either takes a few seconds or forever. Well, I killed
the query after quite some time.

Michael
-- 
Michael Meskes
Michael@Fam-Meskes.De
Go SF 49ers! Go Rhein Fire!
Use Debian GNU/Linux! Use PostgreSQL!

Michael Meskes <meskes@postgresql.org> writes:
> I was just approached with an optimizer question I cannot answer. Does
> our optimizer know how expensive different comparisons are?

It does not, because there's noplace it could look to find the info.

> If there are different comparisons in a where clause check the ints
> first, then the strings, then everything with regexp, or like clauses
> etc. and finally function calls at last, because in most cases a
> function call is the most expensive one. 

Unfortunately, these all look like function calls to the optimizer.

I have played with the notion of forcing subquery-based WHERE clauses
to the end of the list, but that's about all that we could easily manage
without great leaps of assumptions...
        regards, tom lane