Question

I’ve been evaluating PostgreSQL’s hstore functionality (9.2) and the only thing the fine manual isn’t explicit about is how to rename keys. For instance, how could I rename the key c to ai_count?

"c"=>"3", "ai_voltage"=>"3", "ai_temperature"=>"28"

I’m thinking there is no direct way to do this and that it involves duplicating the c key to a ai_count key, then dropping the c key. How can I do this, ideally as a one-liner which can be applied to multiple records?

Answer 1

I think you’re right that you have to pull the old pair out and put the new pair (with the renamed key) back in.

You could do it with a one-liner:

(h - from_key) || hstore(to_key, h -> from_key)

where h is the hstore, from_key is the key you want to change and to_key is what you want to change it to. That will return a new hstore with the desired change but it assumes that from_key is in h; if from_key isn’t in h then you’ll end up with a to_key -> NULL in your hstore. If you, like all sane people, don’t want the stray NULL then I’d wrap the logic in a simple function to make it easier to add an existence check; something like this:

create or replace function
change_hstore_key(h hstore, from_key text, to_key text) returns hstore as $$
begin
    if h ? from_key then
        return (h - from_key) || hstore(to_key, h -> from_key);
    end if;
    return h;
end
$$ language plpgsql;

Then you can say both of these and get the expected results:

=> select change_hstore_key('a=>1,b=>2,c=>3'::hstore, 'b', 'pancakes');
      change_hstore_key       
------------------------------
 "pancakes"=>"2", "a"=>"1", "c"=>"3"

=> select change_hstore_key('a=>1,b=>2,c=>3'::hstore, 'pancakes', 'X');
      change_hstore_key       
------------------------------
 "a"=>"1", "b"=>"2", "c"=>"3"