For users who had disabled or turned off post revisions tracking or versions history feature in WordPress (added since WordPress 2.6), it is also made sense to delete and remove all existing stored post revisions and changes made on pages stored in the database in order to reduce the wp_posts table size, especially when there is already tons of revisions or changes been kept.
To delete and remove all existing post revisions entries and rows from WordPress database Posts table, simply login to MySQL command-line interface, phpMyAdmin, SQLyog or other MySQL GUI tools. Select the appropriate WordPress database (if you have more than one databases on the same server), and then issue the following command (it’s also recommended to backup the database before performing the deletion SQL statements):
DELETE FROM wp_posts WHERE post_type = "revision";
Once all post revisions related records is purged, all revision histories is deleted, and users no longer able to check the changes by phase or compare differences between versions. Thus, deletion may also been used by administrator to ensure privacy or stop authors or writers in the blog from knowing their posts have been edited.