[postgis-devel] KNN and 9.5

[Paragon Corporation]

Steve,

 

Haven't had a chance to look thru what you have here and test it out myself. 

 

The issue I flagged in the ticket was fixed in PostgreSQL 9.5 code base upstream:

 

http://www.postgresql.org/message-id/55630D9D.3090508@iki.fi

 

It's my understanding that you should never get that notice again.  So I'm a bit concerned you are.  It's possible you found another area where this issue arises and is not handled.

 

Are you running PostgreSQL 9.5RC1 and latest PostGIS 2.2 branch (or 2.2.0?).

 

Thanks,

Regina

 

From: postgis-devel [mailto:postgis-devel-bounces at lists.osgeo.org] On Behalf Of Stephen Mather
Sent: Sunday, January 03, 2016 1:56 AM
To: PostGIS Development Discussion <postgis-devel at lists.osgeo.org>
Subject: [postgis-devel] KNN and 9.5

 

Hi All,

 

Short version:

Does Regina's comment on http://trac.osgeo.org/postgis/ticket/2703 regarding ERROR: index returned tuples in wrong order apply to any uses of indices in conjunction with KNN?

 

Long version:

Digging many of the 2.2 / 9.5 updates. Having fun on break playing with skeleton simplification and using KNN in the process. Yes. For those of you who know me, this is an obsession that won't die quietly.

 

https://smathermather.files.wordpress.com/2016/01/screen-shot-2016-01-03-at-1-19-26-am.png

 

I've got a function that does my KNN in the usual style, finds the nearest 2 points, and makes a line from them:

 

CREATE OR REPLACE FUNCTION zz_knn_wonderful_points (geometry) RETURNS geometry AS $$

 

-- Here are my wonderful points to KNN search:

WITH index_query AS (

 

            SELECT edge.geom AS geom

            FROM (SELECT * FROM regions_dpoints WHERE gid = 882) AS edge

-- This is my query point

            ORDER BY $1

                        <->

            edge.geom LIMIT 2

            )

SELECT ST_MakeLine(geom) FROM index_query

 

$$ LANGUAGE SQL;

 

The intent is to implement angle values as an importance criterion for simplifying the skeleton as described at minute 2:20 in Balint Miklos video on the scale axis transform: http://balintmiklos.com/scale-axis/The_Scale_Axis_Picture_Show.mp4

 

Actually, in the final function, I will likely directly calculate the angle and return that instead of the geometry, but I digress.

 

Regardless, when I use this function as is, it works great in drawing lines connecting the two nearest points to the vertex in the medial axis:

 

DROP TABLE IF EXISTS test CASCADE;

CREATE TABLE test AS

WITH returnline AS (

            SELECT oid, gid,

                        ST_Union(

                        ST_MakeLine( ST_StartPoint(zz_knn_wonderful_points(subset.geom)), subset.geom),

                        ST_MakeLine( ST_EndPoint(zz_knn_wonderful_points(subset.geom)), subset.geom)

                        )

                        AS geom FROM

                        (SELECT * FROM regions_points WHERE oid = 882) subset

)

SELECT * FROM returnline

;

 

However, as implemented, this will not work in the case of overlapping geometries, such as Natural Earth's label polygon dataset "ne_10m_geography_regions_polys":

 

https://smathermather.files.wordpress.com/2016/01/screen-shot-2016-01-03-at-1-33-01-am.png

 

And so I add a constraint using an integer ID:

 

CREATE OR REPLACE FUNCTION zz_knn(integer,geometry) RETURNS geometry AS $$

 

-- Here are my wonderful points to KNN search:

WITH index_query AS (

 

            SELECT edge.geom AS geom

            FROM (SELECT * FROM regions_dpoints WHERE gid = $1) AS edge

-- This is my query point

            ORDER BY $2

                        <->

            edge.geom LIMIT 2

            )

SELECT ST_MakeLine(geom) FROM index_query

 

$$ LANGUAGE SQL;

 

And now it takes much longer and fails with 

 

"

ERROR:  index returned tuples in wrong order

CONTEXT:  SQL function "zz_knn" statement 1

********** Error **********

 

ERROR: index returned tuples in wrong order

SQL state: XX000

Context: SQL function "zz_knn" statement 1

"

 

Thanks!

Cheers,

Best,

Steve Mather

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