Postgres 15

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create table mymodel(id,extra_data)as values
 (1,'{"key1":1,"key2":2}'::jsonb)
,(2,'{"key3":3,"key4":4}');

SELECT 2

select id,jsonb_path_query_array(extra_data,'$.keyvalue().key')
from mymodel;

id
jsonb_path_query_array

1
["key1", "key2"]

2
["key3", "key4"]

SELECT 2

select id,array_agg(k)
from mymodel
cross join jsonb_object_keys(extra_data)as ok(k)
group by id
order by id;

id
array_agg

1
{key1,key2}

2
{key3,key4}

SELECT 2