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?.

SELECT ST_AsText(
	ST_Intersection(
		ST_GeomFromText('LINESTRING(632089.750744759 494497.266400368,632103.021656775 494445.107106351)'),
		ST_GeomFromText('LINESTRING(632089.750744759 494497.266400368,632097.05179906 494468.570720864)')
	)
);


st_astext


POINT(632089.750744759 494497.266400368)


SELECT ST_AsText(
	ST_Intersection(
		ST_Snap(
			ST_GeomFromText('LINESTRING(632089.750744759 494497.266400368,632103.021656775 494445.107106351)'),
			ST_GeomFromText('LINESTRING(632089.750744759 494497.266400368,632097.05179906 494468.570720864)'),
			0.001
		),
		ST_GeomFromText('LINESTRING(632089.750744759 494497.266400368,632097.05179906 494468.570720864)')
	)
);


st_astext


LINESTRING(632089.750744759 494497.266400368,632097.05179906 494468.570720864)


SELECT ST_AsText(
	ST_Intersection(
		ST_Snap(
			ST_GeomFromText('LINESTRING(632097.05179906 494468.570720864,632105.975288636 494433.498307163)'),
			ST_GeomFromText('LINESTRING(632089.750744759 494497.266400368,632100.343463809 494455.633335008)'),
			0.00001
		),
			ST_GeomFromText('LINESTRING(632089.750744759 494497.266400368,632100.343463809 494455.633335008)')
	)
);


st_astext


POINT(632100.343463809 494455.633335008)


WITH ix_points AS (
	SELECT ST_Intersection(
		ST_Snap(
			ST_GeomFromText('LINESTRING(632097.05179906 494468.570720864,632105.975288636 494433.498307163)'),
			ST_GeomFromText('LINESTRING(632089.750744759 494497.266400368,632100.343463809 494455.633335008)'),
			0.01
		),
			ST_GeomFromText('LINESTRING(632097.05179906 494468.570720864,632105.975288636 494433.498307163)')
	) AS geom
)
SELECT ST_AsText(
	ST_Split(
		ST_Snap(
			ST_GeomFromText('LINESTRING(632089.750744759 494497.266400368,632100.343463809 494455.633335008)'),
			geom,
			0.01
		),
		geom
	)
) FROM ix_points;


st_astext


GEOMETRYCOLLECTION(LINESTRING(632097.05179906 494468.570720864,632100.343463809 494455.633335008),LINESTRING(632089.750744759 494497.266400368,632097.05179906 494468.570720864))