[postgis-tickets] [PostGIS] #2168: ST_Distance is not always commutative

[PostGIS]

#2168: ST_Distance is not always commutative
---------------------------+------------------------------------------------
 Reporter:  realityexists  |       Owner:  pramsey      
     Type:  defect         |      Status:  new          
 Priority:  medium         |   Milestone:  PostGIS 2.1.0
Component:  postgis        |     Version:  trunk        
 Keywords:                 |  
---------------------------+------------------------------------------------
 I've found a case where changing the order of operands for
 ST_Distance(geog) causes it to return slightly different results:
 {{{
 SELECT ST_Distance('POINT(-40 46.167)'::geography, 'POINT(0
 7.167)'::geography)
         - ST_Distance('POINT(0 7.167)'::geography, 'POINT(-40
 46.167)'::geography);
 }}}

 returns 9.31322574615479e-010 on r10952, Windows x64, PostgreSQL 9.1.4 and
 9.2.1. On 32-bit Linux it returns 0 as expected.

 This caused some odd behaviour in my application, which orders sets of
 points by their distance from each other - the result was different
 depending on the order in which points were considered. There's an easy
 workaround, of course - just round the results - but I can imagine other
 developers scratching their heads over this as well.

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Ticket URL: <https://trac.osgeo.org/postgis/ticket/2168>
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