[postgis-devel] [PostGIS] #1828: Estimate returned by geography_gist_selectivity results in slow query plan for ST_DWithin

[PostGIS]

#1828: Estimate returned by geography_gist_selectivity results in slow query plan
for ST_DWithin
---------------------------+------------------------------------------------
 Reporter:  realityexists  |       Owner:  mcayland     
     Type:  defect         |      Status:  new          
 Priority:  medium         |   Milestone:  PostGIS 2.0.2
Component:  postgis        |     Version:  2.0.x        
 Keywords:                 |  
---------------------------+------------------------------------------------

Comment(by pramsey):

 OK, back looking at this *particular* problem, avoiding thinking about
 handling <func>, just trying to get the selectivity in this case working
 better...

 First question: can we just avoid returning an answer when we are in a
 "don't know" situation. Answer: No. The caller is expecting a valid
 number, and will error out if we don't deliver (plancat.c:1033):
 {{{
         if (result < 0.0 || result > 1.0)
                 elog(ERROR, "invalid restriction selectivity: %f",
 result);
 }}}

 Second question: surely the PgSQL planner has to deal with unknowns! What
 do they do? Answer: Well, there's an example one caller up from plancat.c,
 in (clausesel.c:683)
 {{{

         else if (is_funcclause(clause))
         {
                 /*
                  * This is not an operator, so we guess at the
 selectivity. THIS IS A
                  * HACK TO GET V4 OUT THE DOOR.  FUNCS SHOULD BE ABLE TO
 HAVE
                  * SELECTIVITIES THEMSELVES.       -- JMH 7/9/92
                  */
                 s1 = (Selectivity) 0.3333333;
         }
 }}}
 So, they return a mid-range value when they don't know, presumably to let
 other extreme values one way or the other weigh in first.

-- 
Ticket URL: <http://trac.osgeo.org/postgis/ticket/1828#comment:48>
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