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 tmpem122 (_OGR_GEOMETRY_ geometry(multipolygon,4326));

INSERT INTO tmpem122 VALUES ('SRID=4326;MULTIPOLYGON (((30 20, 45 40, 10 40, 30 20)),((15 5, 40 10, 10 20, 5 10, 15 5)))');

CREATE TABLE

INSERT 0 1

ALTER TABLE tmpem122 ALTER COLUMN _ogr_geometry_ 
TYPE geometry(polygon, 4326) USING ST_GeometryN(_ogr_geometry_,1);

ALTER TABLE

SELECT ST_AsText(_ogr_geometry_) FROM tmpem122;

st_astext

POLYGON((30 20,45 40,10 40,30 20))

SELECT 1