I want to add 2 more rows (for voting up or down) to my default wordpress posts table. Would those rows be deleted when I update my WP version, or cause any other problem? An alternative would be to create a separate table but it's much faster and easier to query from 1 in my case.
First, I assume you're referring to columns, not rows. To answer your question, no, the upgrade process will not remove the extra columns from the posts table. That said, WordPress has the wp_postmeta table that should be used to store extra data about posts rather than adding columns to the posts table. I would suggest using the postmeta table instead of modifying the core database.