> However, this results in an awful slow plan (requiring to scan the complete big_table which obviously isn't optimal)You mean to say there is a sequential scan ? An explain would be helpful. Are there indexes on the provided where clauses. Postgres can do a Bitmap heap scan to combine indexes, there is no need to fire two separate queries.
pgsql-performance by date:
Соглашаюсь с условиями обработки персональных данных
