Very very long query on postmeta table

Hi guys, i’m running an old version of wp ( 3.3.2 ) that will be updated to the last one.

Sometimes it happens that this query is executed

SELECT post_id, meta_key, meta_value FROM wp_1_postmeta WHERE post_id IN( <about 20000 ids> )

and consequentially the server shuts down. I can’t figure out where this query is launched. I just found out that all those ids are attachments and attachments data.

Do you know what could be the cause of this long query?

[Please don’t bump]