Adding a key to an empty hstore column

Question

According to the postgres documentation, you add a key to an hstore column as follows:

UPDATE tab SET h = h || (\'c\' => \'3\');

But it seem

Answer 1

To avoid this you need to ensure the hstore is created as empty and not null. You can add an empty hstore to an existing table:

ALTER TABLE htest ADD h HSTORE NOT NULL DEFAULT '';

Or you can alter your existing hstore to empty:

ALTER TABLE htest ALTER COLUMN h SET NOT NULL;
ALTER TABLE htest ALTER COLUMN h SET DEFAULT '';

Please note that exiting values cannot be null when you set the column to 'NOT NULL'.

Answer 2

For Postgres version > 9.1:

UPDATE htest SET h = COALESCE(h, hstore('')) || hstore('foo', 'bar') WHERE t='key';

Answer 3

I think the problem here is that the hstore you have is null, and null OR some hstore is null.

The best solution I have, which is probably not the best solution, is to make the table with a default empty hstore rather than allowing null. Then your examples work as you would like:

postgres=# create table htest (t text, h hstore default hstore(array[]::varchar[]));
CREATE TABLE
postgres=# insert into htest (t) values ('key');
INSERT 0 1
postgres=# update htest set h = h || ('foo'=>'bar') where t='key';
UPDATE 1
postgres=# select * from htest;
  t  |      h       
-----+--------------
 key | "foo"=>"bar"
(1 row)

I unfortunately do not see a cleaner way to create an empty hstore than hstore(array[]::varchar[]) but that doesn't mean there isn't a better way. You could incorporate this into your hstore update from before like so:

update htest set h = coalesce(h, hstore(array[]::varchar[])) || ('foo'=>'bar') where t='key';

This way you don't need to recreate the table. I find that fairly gross though. Hope this helps.

Answer 4

how about:

UPDATE htest SET h = COALESCE(h, '') || ('foo'=>'bar') WHERE t='key';