In case of map-joins, it is likely that the big table will not find many matching rows from the small table.
Currently, we perform a hash-map lookup for every row in the big table, which can be pretty expensive.
It might be useful to try out a bloom-filter containing all the elements in the small table.
Each element from the big table is first searched in the bloom filter, and only in case of a positive match,
the small table hash table is explored.