Postgres 14

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CREATE  TABLE tbl (the_column JSON);
INSERT INTO tbl VALUES
  ('{"evens": [2, 4, 6]}')
, ('{"odds": [1, 3, 5]}')
, ('{"multi1": [11, 13, 15], "multi2": [12, 14, 16]}')  -- added to highlight difference
;

CREATE TABLE

INSERT 0 3

-- build array of objects
SELECT json_agg(the_column) AS result
FROM   tbl;

result

[{"evens": [2, 4, 6]}, {"odds": [1, 3, 5]}, {"multi1": [11, 13, 15], "multi2": [12, 14, 16]}]

SELECT 1

-- build object with all unnested keys
SELECT json_object_agg(key, value) AS result
FROM   tbl, json_each(the_column);

result

{ "evens" : [2, 4, 6], "odds" : [1, 3, 5], "multi1" : [11, 13, 15], "multi2" : [12, 14, 16] }

SELECT 1