Postgres not using indexes

We have a huge performance issues in Postgres that surfaced due to existing indexes not being used like in the example below in both 8.35 and 9.0 versions.

Client_Orders table with and int ID as PK which is the order_id and indexed - about 155,000 rows
Order_Items table with and int ID primary key and INDEX on Order_id (int) matching the ID in the above client_orders table. - about 33 million rows

A query like below takes almost ten minutes to complete however the result set is 33768 rows and Total query runtime: 427539 ms.!!! This is due to the fact that the index on Order_Items it is NOT used and a sequence scan is done instead but this is obviously not acceptable from performance point of view. If I add a LIMIT 1000 for instance then the index is used and query returns results in no time as expected but as soon as I go higher in the limit to a few thousands then the index on Order_Items.Order_id is no longer used - why??? Is there any way to force Postgres to use the existing indexes instead of table seq scan which is deadly?

select oi.id from order_items oi INNER JOIN client_orders co ON oi.order_id = co.id

Regards,
Nenea Nelu.

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