Re: KNN-GiST with recheck

On Fri, Sep 26, 2014 at 10:49:42AM +0400, Alexander Korotkov wrote:
> Does this also fix the identical PostGIS problem or is there something
> PostGIS needs to do?
>
>
> This patch provides general infrastructure for recheck in KNN-GiST. PostGIS
> need corresponding change in its GiST opclass. Since PostGIS already define <->
> and <#> operators as distance to bounding box border and bounding box center,
> it can't change their behaviour.
> it has to support new operator "exact distance" in opclass. 

Ah, OK, so they just need something that can be used for the recheck. I
think they currently use ST_Distance() for that. Does it have to be an
operator? If they defined an operator for ST_Distance(), would
ST_Distance() work too for KNN-GiST?

In summary, you still create a normal GiST index on the column:

http://shisaa.jp/postset/postgis-postgresqls-spatial-partner-part-3.html

CREATE INDEX planet_osm_line_ref_index ON planet_osm_line(ref);

which indexes by the bounding box. The new code will allow ordered
index hits to be filtered by something like ST_Distance(), rather than
having to a LIMIT 50 in a CTE, then call ST_Distance(), like this:

EXPLAIN ANALYZE WITH distance AS (
SELECT way AS road, ref AS route
FROM planet_osm_line
WHERE highway = 'secondary'
ORDER BY ST_GeomFromText('POLYGON((14239931.42 3054117.72,14239990.49 3054224.25,14240230.15 3054091.38,14240171.08 3053984.84,14239931.42 3054117.72))', 900913) <#> way
LIMIT 50
)
SELECT ST_Distance(ST_GeomFromText('POLYGON((14239931.42 3054117.72,14239990.49 3054224.25,14240230.15 3054091.38,14240171.08 3053984.84,14239931.42 3054117.72))', 900913), road) AS true_distance, route
FROM distance
ORDER BY true_distance
LIMIT 1;

Notice the CTE uses <#> (bounding box center), and then the outer query
uses ST_Distance and LIMIT 1 to find the closest item.

Excellent!

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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