I’d previously closed pinging for all ancient posts to prevent trackback spam, but after installing Akismet my blogs have ceased to suffer from it, so I decided it was time to re-open them.
Took me some time to find the appropriate SQL spells, but I finally came across the right ones at WP’s support forums: by applying btvillarin’s SQL instructions I was able to achieve somewhat the reverse of what he was aiming at:
SET ping_status = "open"
WHERE ping_status = "closed";
This causes all posts with their ping status currently closed be changed so that they’re once again open for pings.
Note that you might have to replace wp_posts in the above with yourtableprefix_posts, in case you have changed the default table prefix.