join and query planner

(first at all, sorry for my english)
Hi.
   - Does "left join" restrict the order in which the planner must join
tables? I've read about join, but i'm not sure about left join...
   - If so: Can I avoid this behavior? I mean, make the planner resolve the
query, using statistics (uniqueness, data distribution) rather than join
order.

	My query looks like:
	SELECT ...
      FROM a, b,
      LEFT JOIN c ON (c.key = a.key)
      LEFT JOIN d on (d.key=a.key)
      WHERE (a.key = b.key)  AND (b.column <= 100)

      b.column has a lot better selectivity, but planner insist on resolve
first c.key = a.key.

	Of course, I could rewrite something like:
	SELECT ...
      FROM
           (SELECT ...
            FROM a,b
            LEFT JOIN d on (d.key=a.key)
            WHERE (b.column <= 100)
            )
            as aa
      LEFT JOIN c ON (c.key = aa.key)

	but this is query is constructed by an application with a "multicolumn"
filter. It's dynamic.
      It means that a user could choose to look for "c.column = 1000". And
also, combinations of filters.

	So, I need the planner to choose the best plan...

I've already change statistics, I clustered tables with cluster, ran vacuum
analyze, changed work_mem, shared_buffers...

Greetings. TIA.