Set 1st image of old published posts as featured image

Set 1st image of old published posts as featured image

I am moving my WordPress website to a new theme and the new theme uses
featured images where as old one didnt hence my old posts dont have any
featured images. Now there are over 3000 posts in my website and obviously
setting featured images for them manually is kind of impossible.
So can you guys help me make an SQL query or link me to some plugin which
can do this?
There are a few requirements if these can be fulfilled easily. First of
all, I want the first image of the post to be set as featured not the
first 'attached' images. Secondly, the query or plugin shouldnt do
anything to posts which already have featured images.
Thanks,