sessions table cleanup ?

[Thomas Guthmann]

Hey guys,

We are running our own private koji instance. We are running 1.6 on el5.
Lately this query takes 20mins or so to execute :

     SELECT host.id,name,arches,task_load,capacity FROM host
         JOIN sessions USING (user_id)
     WHERE enabled = TRUE AND ready = TRUE
         AND expired = FALSE
         AND master IS NULL
         AND update_time > NOW() - '5 minutes'::interval

It looks like the 'sessions' table is the culprit. Indeed SELECTing 
'host' is immediate whereas SELECT count(id) from 'sessions' takes 15 
seconds for only 408312 rows... looks like a vacuum problem you would 
say. You are right but my question is why do I have so many sessions 
rows ? Can 'sessions' be truncated ? I am just wondering if it's a known 
issue to not clean the sessions tables or if I need to tune my 
autovacuum to work properly :)

Cheers,
Thomas