Multiple-Table-Spanning Joins with ORs in WHERE Clause

Hi pgsql-performance list,

what is the recommended way of doing **multiple-table-spanning joins
with ORs in the WHERE-clause**?

Until now, we've used the LEFT OUTER JOIN to filter big_table like so:

SELECT DISTINCT <fields of big_table>
FROM
"big_table"
LEFT OUTER JOIN "table_a" ON ("big_table"."id" =
"table_a"."big_table_id")
LEFT OUTER JOIN "table_b" ON ("big_table"."id" =
"table_b"."big_table_id")
WHERE
"table_a"."item_id" IN (<handful of items>)
OR
"table_b"."item_id" IN (<handful of items>);

However, this results in an awful slow plan (requiring to scan the
complete big_table which obviously isn't optimal).
So, we decided (at least for now) to split up the query into two
separate ones and merge/de-duplicate the result with application logic:

SELECT <fields of big_table>
FROM
"big_table" INNER JOIN "table_a" ON ("big_table"."id" =
"table_a"."big_table_id")
WHERE
"table_a"."item_id" IN (<handful of items>);

SELECT <fields of big_table>
FROM
"big_table" INNER JOIN "table_b" ON ("big_table"."id" =
"table_b"."big_table_id")
WHERE
"table_b"."item_id" IN (<handful of items>);

As you can imagine we would be very glad to solve this issue with a
single query and without having to re-code existing logic of PostgreSQL.
But how?

Best,
Sven

PS: if you require EXPLAIN ANALYZE, I can post them as well.