Postgres 12

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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

CREATE TABLE building (name text, long numeric, lat numeric);
INSERT INTO building VALUES ('Kirk Michael',-4.5896,54.2835);
INSERT INTO building VALUES ('Baldrine',-4.4077,54.2011);
INSERT INTO building VALUES ('Isle of Man Airport',-4.6283,54.0804);

CREATE TABLE

INSERT 0 1

INSERT 0 1

INSERT 0 1

SELECT name,long,lat,
  ST_Distance('POINT(-4.6314 54.0887)'::geography,
              ST_MakePoint(long,lat)) * 0.000621371 AS distance
FROM building
WHERE
  ST_DWithin('POINT(-4.6314 54.0887)'::geography,
              ST_MakePoint(long,lat),8046.72); -- 8046.72 meters = 5 miles;

name
long
lat
distance

Isle of Man Airport
-4.6283
54.0804
0.5877283470621737

SELECT 1

SELECT name,long,lat,
  ST_Distance('POINT(-4.6314 54.0887)'::geography,
              ST_MakePoint(long,lat)) * 0.000621371 AS distance
FROM building
WHERE 
  ST_Distance('POINT(-4.6314 54.0887)'::geography,
              ST_MakePoint(long,lat)) * 0.000621371 <= 5;

name
long
lat
distance

Isle of Man Airport
-4.6283
54.0804
0.5877283470621737

SELECT 1