jboes@xxxxxxxxxxxx (Jeff Boes) writes: > (Environment: Pg 7.4.1 on Linux; yes, I know it's past time to > upgrade.) If you were running 7.2, I'd be pestering you ;-). We didn't get rid of our last 7.1 system until last December, and I think the backend is probably still running, albeit with nothing hooking up to it :-). > (Originally, I started to post this as a question about how > pg_class.reltuples could get so far out of whack compared to the > actual table size. After posting it, I found that (a) I had the > mailing list address wrong [d'oh!], and (b) I think I solved my > problem. In essence, by doing a VACUUM in a script after a sizeable > DELETE, apparently I took the affected table out of the hands of > pg_autovacuum so that it never, ever did anything with the > table. Including ANALYZE. Thus, reltuples never got updated.) Oops. Change that to a VACUUM ANALYZE (in your script), and things should work out better. > I started tracking the COUNT(*) (actual row count) versus the value > in pg_class.reltuples for a number of our larger tables. Some of the > tables see a lot of INSERTs and DELETEs over the course of a day; as > much as 1/12th of the data will be deleted overnight, and new data > inserted over the course of the day. I have pg_autovacuum running, > and I also do regular VACUUMs, ANALYZEs, and even some CLUSTERs on > these tables. > > [N.B.: In fact, I started doing a VACUUM explicitly after the big > nightly DELETE.] That's not a bad idea, all in all. > What would cause an otherwise well-behaved table to start doing > this? Is this just a "dead spot" in the ANALYZE command? (By which I > mean: ANALYZE randomly sampling rows, but my data is not terribly > random, so it gets fooled?) Yes, that doubtless would be it. There's a new sampling scheme in version 8 that may do better. > [And here's the remaining question in my puzzled mind: ANALYZE would > not change the reltuples value, but VACUUM FULL ANALYZE > did. Er-wha?] VACUUM ANALYZE would also change the reltuples value... -- (format nil "~S@~S" "cbbrowne" "acm.org") http://www.ntlug.org/~cbbrowne/sap.html Rules of the Evil Overlord #78. "I will not tell my Legions of Terror "And he must be taken alive!" The command will be: ``And try to take him alive if it is reasonably practical.''" <http://www.eviloverlord.com/>
