PostGIS minimum distance between two large sets of points

Question

I have two tables of points in PostGIS, say A and B, and I want to know, for every point in A, what is the distance to the closest point in B. I am able to solve this for sm

Answer 1

Your query is slow because it computes the distance between every points without using any index. You could rewrite it to use the <-> operator that uses the index if used in the order by clause.

select a.id,closest_pt.id, closest_pt.dist
from tablea a
CROSS JOIN LATERAL
  (SELECT
     id , 
     a.geom <-> b.geom as dist
     FROM tableb b
     ORDER BY a.geom <-> b.geom
   LIMIT 1) AS closest_pt;