I have the same problem in PS 1.7.6, and i can't find how to do it from the database (ps_product_attribute.default_on doesn't change anything)
EDIT :
It works, and to make it work, you have to set it in tables fields :
ps_product.cache_default_attribute
ps_product_shop.cache_default_attribute
ps_product_attribute.default_on
ps_product_attribute_shop.default_on