How do WP know an image is a post thumbnail

Question

What is stored in the WP database to let WP know I uploaded an image as a post thumbnail? I'm looking at the wp_posts table but I can't find anything.

Answer 1

It' stored in the post meta table as _thumbnail_id.

The thumbnail is actually a "post" entry too, I think it's stored as an "attachment" post type. So the _thumbnail_id meta value will be the ID which points to that attachment.