SELECT for UPDATE and outer join?

Folks,

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SQL statement<select t0.po_number, t0.po_site_id, t0.po_supp_id,
t1.pol_number,
t1.pol_po_id, t1.pol_balance, t1.pol_qty, t1.pol_p_id, t1.pol_message,
t1.pol_de
ldate from  s_purchase_order t0  left outer join s_purchase_orderline
t1  on t0.
po_number = t1.pol_po_id   where t0.po_number = ? for update > with
input values
:java.lang.Integer:30239.
Please examine the SQLException for more information.
NestedException: java.sql.SQLException: ERROR: SELECT FOR UPDATE cannot
be applied to the nullable side of an outer join
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So, my question is: why can't we apply FOR UPDATE to an outer join?   Is this
defined in the SQL Spec?

--
Josh Berkus
Aglio Database Solutions
San Francisco