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 seems to only work if the hstore field is not empty. For example:

postgres=# create table htest (t text, h hstore);
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 | 
(1 row)

The update was successful, yet the hstore was not updated. However:

postgres=# update htest set h = ('foo'=>'bar') where t='key';
UPDATE 1
postgres=# select * from htest;
  t  |      h       
-----+--------------
 key | "foo"=>"bar"
(1 row)

postgres=# update htest set h = h || ('bar'=>'foo') where t='key';
UPDATE 1
postgres=# select * from htest;
  t  |             h              
-----+----------------------------
 key | "bar"=>"foo", "foo"=>"bar"
(1 row)

Is there a way to atomically add a key to an hstore without first checking if the hstore is empty?

Answer 1

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.