"plain old SQL" can require a lot of mangling one's data to fit its constraints. I refuse to believe that there isn't a better key-value store for the case where the values are json documents, even if mongodb isn't it.
create table keyvalue (key text, value text);
create index keyvalue_idx on keyvalue(key);
insert into keyvalue (key, value) values ('key', 'value');
select value from keyvalue where key = 'key';