Column default

Hi All,

I am having difficulty understanding the use of defaults.

I have a table with defaults defined in three columns.  When I run a function
that inserts/or updates records the default value is not written into the
record.  I thought this was a result of updates occurring and not inserts.
So, I changed the source table to have the same defaults.  However when I use
the COPY command to place data in the source table the defaults do not appear
to be applied there either.

When I run a query on the table specifying WHERE ptos_uom_factor ISNULL I get
lots of records back.  I though that using a default would prevent this outcome.

When is the default value applied?  On INSERT? UPDATE? COPY? None of the above?

TIA

        Column        |          Type          |            Modifiers

----------------------+------------------------+--------------------------------
-
 id                   | character varying(20)  | not null
 description          | character varying(30)  | not null
 item_class           | smallint               | not null
 inactive             | boolean                | not null
 sales_description    | character varying(160) |
 purchase_description | character varying(160) |
 last_unit_cost       | real                   | not null
 costing_method       | smallint               | not null
 sales_gl_account     | character varying(15)  |
 inventory_gl_account | character varying(15)  |
 cogs_gl_account      | character varying(15)  |
 item_type            | character varying(8)   |
 unit_of_measure      | character varying(6)   | default 'ea'::character varying
 weight               | real                   |
 reorder_point        | real                   |
 reorder_quantity     | real                   |
 purchase_uom         | character varying(6)   | default 'ea'::character varying
 ptos_uom_factor      | real                   | default 1


Kind Regards,
Keith