pgsql-bugs(at)postgresql(dot)org writes:
> I have problems with moving cursors on joined tables.
Most of the more complex plan node types don't really support moving
backwards. I suppose it'd be an idea to error out rather than silently
doing the wrong thing. Fixing the code to actually do the right thing
is way down the priority list, unfortunately.
As a workaround you could modify the query to force a sort step after
the join, eg with an ORDER BY that doesn't match the join condition.
A sort node can cope with backwards scan of its output ...
regards, tom lane