Optimize SELECT * from table WHERE foreign_key_id IN (key1, key2, key3, key4...)

Hi,

I'm running a rails app, where I have a model called Car that has_many Images. Now when I tell rails to include those images, when querying say 50 cars, then it often decides to use a SELECT * from images WHERE car_id IN (id1,id2,id3,id4…) instead of doing a join.

Now either way it uses the index I have on car_id:

Index Scan using car_id_ix on adverts (cost=0.47..5665.34 rows=1224 width=234)
Index Cond: (car_id = ANY ('{7097561,7253541,5159633,6674471,...}'::integer[]))

But it's slow, it's very slow. In this case it took 3,323ms

Can I do anything to optimize that query or maybe the index or something?

The table has 16.000.000 rows