Re: Planner hints in Postgresql

On 3/17/14, 5:12 PM, Claudio Freire wrote:
>
> On Mon, Mar 17, 2014 at 7:09 PM, Jim Nasby <jim(at)nasby(dot)net <mailto:jim(at)nasby(dot)net>> wrote:
>
> On 3/17/14, 2:16 PM, Merlin Moncure wrote:
>
> On Mon, Mar 17, 2014 at 1:45 PM, Pavel Stehule<pavel(dot)stehule(at)gmail(dot)__com <mailto:pavel(dot)stehule(at)gmail(dot)com>> wrote:
>
> >I don't believe so SELECTIVITY can work well too. Slow queries are usually
> >related to some strange points in data. I am thinking so well concept should
> >be based on validity of estimations. Some plans are based on totally wrong
> >estimation, but should be fast due less sensitivity to bad estimations. So
> >well concept is penalization some risk plans - or use brute force - like
> >COLUMN store engine does. Their plan is usually simply and tolerant to bad
> >estimations.
>
> Disagree. There is a special case of slow query where problem is not
> with the data but with the expression over the data; something in the
> query defeats sampled selectivity. Common culprits are:
>
> *) CASE expressions
> *) COALESCE
> *) casts
> *) simple tranformational expressions
> *) predicate string concatenation
>
>
> *) time/date functions, ie WHERE date_trunc( 'quarter', some_timestamp ) = '2014-1-1'
>
> Though, in this case it's probably much better to teach the parser how to turn that into a range expression.
>
>
>
> Maybe, maybe not.
>
> An index over the truncated time can potentially be much more efficient.

More efficient than a range index? Maybe, but I'm doubtful. Even if that's true, in a warehouse you're going to want to limit by weeks, months, quarters, years, etc. So now you're stuck building tons of special indexes.

(Granted, most warehouses build a separate date dimension because of these kinds of problems... I'm hoping that we could do something better.)
--
Jim C. Nasby, Data Architect jim(at)nasby(dot)net
512.569.9461 (cell) http://jim.nasby.net