WordPress has very useful function to store previous sessions and copies of every post to restore, if you lost your connection or you want to compare your post with another post. Its a good feature but at the same time, it takes a lot of space in your database. Here I have a piece of code to remove unnecessary post revisions.
Please backup your database before executing these codes.
Simply go to you phpMyAdmin > Select your Database >SQL query > Execute the following command and then Go.
FROM wp_posts a
LEFT JOIN wp_term_relationships b ON (a.ID = b.object_id)
LEFT JOIN wp_postmeta c ON (a.ID = c.post_id)
WHERE a.post_type = 'revision'