Question

I’m not even sure that Postgres’ HStore data type can contain nested hashes, and if they can, how to insert them?

Here’s what I’ve tried so far:

-- Database: test1

-- DROP DATABASE test1;
/*
CREATE DATABASE test1
  WITH OWNER = iainuser
       ENCODING = 'UTF8'
       TABLESPACE = pg_default
       LC_COLLATE = 'en_GB.UTF-8'
       LC_CTYPE = 'en_GB.UTF-8'
       CONNECTION LIMIT = -1;
*/
/* create extension hstore; */
/*drop table my_store;*/
/*
create table my_store (
  id serial primary key not null,
  doc hstore
);

CREATE INDEX my_store_doc_idx_gist
  ON my_store
  USING gist
  (doc);
*/
/* select doc from my_store; */
/*
insert into my_store (doc) values ( '"a" => "1"' );
select doc -> 'a' as first_key from my_store; -- returns "1"
*/

/* insert into my_store (doc) values ( '"b" => "c" => "3"' ); -- doesn't work */
/* insert into my_store (doc) values ( '"b" => ("c" => "3")' ); -- doesn't work */
/* insert into my_store (doc) values ( '"b" => hstore("c" => "3")' ); -- doesn't work */
/* insert into my_store (doc) values ( '"b"' => hstore("c" => "3")' ); -- doesn't work */
/* insert into my_store (doc) values ( "b"=>'"c"=>"3"'::hstore ); -- doesn't work */

If it’s not possible, is there a current accepted standard/idiom for working with nested hashes – perhaps pull them apart and refer to them using id’s?

Any help with this would be much appreciated.

Answer 1

From the fine manual:

Keys and values are simply text strings.

So, no, you can’t use an hstore as a value in an hstore. If you look at the hstore operators and functions you’ll see that they all work with text values.

I don’t know of any standard approach to faking nested hashes. I suspect you’d have to structure the keys (a.b => c for a => b => c), then you could so things like this:

select slice(doc, array['a.b', 'a.c'])
from my_store
where doc ?& array['a.b', 'a.c']

to grab the “a” slice of each doc that has the {b => ..., c => ...} “sub-hash”.

There’s also a JSON type coming up that might be better suited to your needs. But, you’ll have to wait for it and I’m not sure what the final implementation will look like.