Я бы сказал, что там всётаки нету индексов, иначе бы небыло Seq Scan. Или индексы не включены.
«For each partition, create an index on the key column(s), as well as any other indexes you might want. (The key index is not strictly necessary, but in most scenarios it is helpful. If you intend the key values to be unique then you should always create a unique or primary-key constraint for each partition.)»
www.postgresql.org/docs/9.1/static/ddl-partitioning.html