From: Paul Ramsey <pramsey@cleverelephant.ca>
Date: Wed, 21 Nov 2012 19:52:41 +0000 (+0000)
Subject: Explain "selectivity" a little more explicitly
X-Git-Tag: 2.1.0beta2~368
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=2868f8cb8fc8a1bbc24eabee2bc007de986e9197;p=postgis

Explain "selectivity" a little more explicitly


git-svn-id: http://svn.osgeo.org/postgis/trunk@10727 b70326c6-7e19-0410-871a-916f4a2858ee
---

diff --git a/postgis/geometry_estimate.c b/postgis/geometry_estimate.c
index 3fa717ade..888119627 100644
--- a/postgis/geometry_estimate.c
+++ b/postgis/geometry_estimate.c
@@ -164,8 +164,12 @@ calculate_column_intersection(GBOX *search_box, GEOM_STATS *geomstats1, GEOM_STA
 }
 
 /**
-* JOIN selectivity in the GiST && operator
-* for all PG versions
+* Join selectivity of the && operator. The selectivity
+* is the ratio of the number of rows we think will be 
+* returned divided the maximum number of rows the join
+* could possibly return (the full combinatoric join).
+*
+* selectivity = estimated_nrows / (totalrows1 * totalrows2)
 */
 PG_FUNCTION_INFO_V1(geometry_gist_joinsel_2d);
 Datum geometry_gist_joinsel_2d(PG_FUNCTION_ARGS)
@@ -202,8 +206,6 @@ Datum geometry_gist_joinsel_2d(PG_FUNCTION_ARGS)
 	* geometry in col 1 that intersects a geometry in col 2, the same
 	* will also be true.
 	*/
-
-
 	POSTGIS_DEBUGF(3, "geometry_gist_joinsel called with jointype %d", jointype);
 
 	/*
@@ -378,15 +380,15 @@ Datum geometry_gist_joinsel_2d(PG_FUNCTION_ARGS)
 
 #endif /* REALLY_DO_JOINSEL */
 
-/**************************** FROM POSTGIS ****************/
 
 
 /**
- * This function returns an estimate of the selectivity
- * of a search_box looking at data in the GEOM_STATS
- * structure.
- * */
-/**
+* This function returns an estimate of the selectivity
+* of a search GBOX by looking at data in the GEOM_STATS
+* structure. The selectivity is a float from 0-1, that estimates
+* the proportion of the rows in the table that will be returned
+* as a result of the search box.
+*
 * TODO: handle box dimension collapses (probably should be handled
 * by the statistic generator, avoiding GEOM_STATS with collapsed
 * dimensions)
@@ -673,8 +675,6 @@ Datum _postgis_geometry_sel(PG_FUNCTION_ARGS)
  * This function just tries to find the search_box, loads the statistics
  * and invoke the work-horse.
  *
- * This is the one used for PG version >= 7.5
- *
  */
 PG_FUNCTION_INFO_V1(geometry_gist_sel_2d);
 Datum geometry_gist_sel_2d(PG_FUNCTION_ARGS)