Re: sql query not use index

From: Maxim Demenko <mdemenko_at_gmail.com>
Date: Mon, 09 Mar 2009 20:42:39 +0100
Message-ID: <49B5712F.2030409_at_gmail.com>

sydneypuente_at_yahoo.com schrieb:
> Hello Guys,
>
> I was asked yesterday a project manager what sort of query will not
> use an index?
> What sort of index ? I asked
> say a varchar(256) ?
> A badly written regex. I responded.
> OK thanks.
>
> Now I am worried I have given a misleading or wrong response!
> Can anyone help me out with a better answer?
>
> TIA
>
> Syd

I'm afraid, this question can't be answered (in sense - there is such sort of queries) in general case - in opposite, i can imagine, every query can be forced to use an index ( it may be of course not the optimal access path), provided an appropriated index inplace. Popular rules of thumbs ( null values are not indexed, selectivity is more than 10%, etc) don't count, because under certain conditions they are not valid. Your project manager may be interested in selfeducating at least on

http://richardfoote.wordpress.com/
http://www.catb.org/~esr/faqs/smart-questions.html

The badly (whatever it supposed to be) written regular expression can perfectly be applied to an index as rowsource...

Best regards

Maxim Received on Mon Mar 09 2009 - 14:42:39 CDT