Postgresql 9.5 JSON problems

There is no way that saves you from specifying all JSON attributes in the select list, whether implicitly through the "dummy type" or explicitly, e.g. using something like this:

select json_column ->> 'text1' as text1, 
       json_column ->> 'text2' as text2, 
       ... 
from the_table;

What you can do, is to make this simpler by automatically creating a view with all attributes based on the distinct attributes in the JSON documents.

The following code will re-create a view with all distinct keys from the JSON column:

do
$$
declare
  l_keys text;
begin
   drop view if exists v_json_view cascade;

   select string_agg(distinct format('json_column ->> %L as %I',jkey, jkey), ', ')
     into l_keys
   from the_table, json_object_keys(json_column) as t(jkey);

   execute 'create view v_json_view as select '||l_keys||' from the_table';
end;
$$
;

You will need to run the above every time the list of keys in all json documents changes. Theoretically this could be done in a trigger but that's probably not a good idea if you run many updates on that table.

If the total number of JSON keys is somewhat "stable", you could schedule a cron job to re-create that view in regular intervals.

You are also limited by the maximum number of columns in a table or view. If you have more (distinct) keys than approx. 1600 (maybe less) the above will fail.

select  array_to_json 
        (
           (select  array_agg(my_db_field->n order by n) 
            from    generate_series(0,least(300,json_array_length(my_db_field))-1) gs(n) 
            )
        )

from    my_db_table

This solution (the original in this answer) most likely does not guarantee the order of the elements

select (array(select json_array_elements(my_db_field)))[1:300]
from    my_db_table