ST_DWITHIN with zero buffer faster than ST_WITHIN ?

[glaroc]

Hi, I'm performing a spatial join with:

CREATE TABLE regions_obs AS
           SELECT adm2_name, count(*) AS cnt
           FROM atlas a
           JOIN regions_qc r ON ST_Within(a.geom, r.geometry) 
           GROUP BY adm2_name;

The atlas table has 55 million rows and an RTREE index on the geometry. The regions_qc file has 17 polygons. The query above takes several hours and a lot of memory on a machine with 15 cores and 180GB of RAM.

However, this query

CREATE TABLE regions_obs AS
           SELECT adm2_name, count(*) AS cnt
           FROM atlas a
           JOIN regions_qc r ON ST_DWithin(a.geom, r.geometry,0)
           GROUP BY adm2_name;

Which, to my understanding, should be identical, finishes in about 15 minutes.

Any idea why that is ?

With Duckdb 1.5.3 and most recent spatial extension commit.