Postgres 12

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CREATE TABLE test_hstore AS
SELECT '1'::int AS id, NULL::hstore AS map;  -- table with 1 row
1 rows affected

TABLE test_hstore;

id
map

1
null

UPDATE test_hstore SET map = map || hstore('key1', 'value1')
RETURNING *;

id
map

1
null

UPDATE test_hstore SET map = COALESCE(map, '') || hstore('key1', 'value1')
RETURNING *;

id
map

1
"key1"=>"value1"