On 23/09/2007, Alvaro Herrera <[EMAIL PROTECTED]> wrote: > > Christian Schröder wrote: > > Alvaro Herrera wrote: > >> Christian Schröder wrote: > >> > >> > >>> I think it is my job as db admin to make the database work the way my > >>> users need it, and not the user's job to find a solution that fits the > >>> database's needs ... > >>> > >>> Is there really nothing that I can do? > >>> > >> > >> You can improve the selectivity estimator function. One idea is that > if > >> you are storing something that's not really a general character string, > >> develop a specific datatype, with a more precise selectivity estimator. > >> If you are you up to coding in C, that is. > >> > > > > Hm, that sounds interesting! I will definitely give it a try. > > Will that also solve the problem of combining more than one of these > > conditions? As far as I can see, the main issue at the moment is that we > > often have "... where test like '11%' and test not like '113%'" in our > > queries. Even if the selectivity estimation of the single condition will > be > > improved, it will still be wrong to multiply the selectivities. > > Unless you can come up with an operator that expresses better the > "starts with 11 but not with 113" type of condition. For example if > these were telephone number prefixes or something like that, probably > there's some way to do that in a single operation instead of two, and > the selectivity function could produce a much more accurate estimate > saving the need to multiply.
select a from b where a ~ '^11[^3]' Is that what you want? I usually find using ~ far better than like..... Peter Childs -- > Alvaro Herrera > http://www.advogato.org/person/alvherre > "I think my standards have lowered enough that now I think 'good design' > is when the page doesn't irritate the living f*ck out of me." (JWZ) > > ---------------------------(end of broadcast)--------------------------- > TIP 1: if posting/reading through Usenet, please send an appropriate > subscribe-nomail command to [EMAIL PROTECTED] so that your > message can get through to the mailing list cleanly >
