How to modify Sort field in Custom Field after data has been inse rted?

I have created a few custom fields in RT3 and entered the data in the order
it is to appear in the list. Now I need to add an additional item to appear
part way through the list. Unfortunately all of my entries have the default
Sort value of ‘0’.

I can’t see a way to modify it in the Web UI other than to delete
everything and reinsert it using the correct sort values. Is it possible
(and safe) to simply update the sort field directly in the appropriate
database table? If so, can someone point me at the right table and field for
a Postgresql install.

Thanks in advance.