Query locking tables and stacking queries - need help understanding

[starl]

Our forum has been lagging a lot at night, even crashing. We've done a lot of hardware changes to manage it, but it's not enough. After this morning's lag, server support told me:

queries had locked tables in the DB, causing the queries after them to stack up until the limit was hit. The query causing the lock at the time of the logging snapshot was showing as follows :

----------------------------------------------------------------

3669084 Query SELECT threadid, title, prefixid, lastpost, replycount
FROM vb_thread AS thread
WHERE forumid = 10
AND visible = 1
AND open <> 10

ORDER BY dateline DESC
LIMIT 182750,250
________________
I get the gist of it: this one query was locking a table that other queries needed, so those queries queued up until the table was unlocked. What I don't get - What is this query? Is this one of the scheduled tasks?

Our forum database is very large. Server support suggests archiving posts, but we dont' want to make posts unsearchabel/unfindable...

note: while our forum has been around a long time, I never was involved in the guts at this level. I am trying to learn and come up to speed as quickly as possible.

thanks