I couldn't find a plug-in, so I assume this must be done by SQL.
What I want to do for every post is copy the post_content value into a custom field Value I created for the same post.
One thing to note is that post_content is in a wp_posts table, and the custom field value is in the wp_postmeta table. So two different tables.
Thanks in advance. Having been looking for a solution for a long time.