Re: Performance Issues

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.

I think I will have a look at the src/backend/optimizer/util/plancat.c,
src/backend/optimizer/path/clausesel.c and
src/backend/utils/adt/selfuncs.c files after my holiday.

Kind regards,
    Christian

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