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 grid_cell_data (
  id int,
  name text,
  geometryOfCellRepresentativeToTreatment geometry(POLYGON,4326)
);

INSERT INTO grid_cell_data VALUES 
(1,'my polygon','SRID=4326;POLYGON ((30 10, 40 40, 20 40, 10 20, 30 10))'::geometry);

CREATE TABLE

INSERT 0 1

SELECT ST_AsMVT(g) FROM (
  SELECT name,geometryOfCellRepresentativeToTreatment
  FROM grid_cell_data) g;

st_asmvt

\x1a370a0764656661756c741213120200001803220b093c141a143c270013270f1a046e616d65220c0a0a6d7920706f6c79676f6e2880207802

SELECT 1

SELECT ST_AsMVT(grid_cell_data) 
FROM grid_cell_data;

st_asmvt

\x1a410a0764656661756c7412151204000001011803220b093c141a143c270013270f1a0269641a046e616d6522022801220c0a0a6d7920706f6c79676f6e2880207802

SELECT 1

SELECT
 ST_AsText(
   ST_AsMVTGeom(
     geometryOfCellRepresentativeToTreatment,
     ST_MakeBox2D(ST_Point(0, 0), ST_Point(4096, 4096))))
FROM grid_cell_data;

st_astext

POLYGON((10 4076,20 4056,40 4056,30 4086,10 4076))

SELECT 1