Re: Planner hints in Postgresql - Mailing list pgsql-hackers

On 3/17/14, 2:16 PM, Merlin Moncure wrote:

On Mon, Mar 17, 2014 at 1:45 PM, Pavel Stehule<pavel.stehule@gmail.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.