Postgres 11

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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 mytable (
    mydata text
);

INSERT INTO mytable VALUES ('["elem1","elem2","elem3"]')
1 rows affected

SELECT 
    to_json(mydata)
FROM mytable

to_json

"["elem1","elem2","elem3"]"

SELECT 
    mydata::json
FROM mytable

mydata

["elem1","elem2","elem3"]

SELECT 
    array_agg(elems)
FROM mytable,
    json_array_elements_text(mydata::json) AS elems

array_agg

{elem1,elem2,elem3}