David Scott <davids@xxxxxxxxxxxxxx> writes: > I didn't mention I was the only user with transactions open on the > system during this. Would cluster eliminate more rows then vacuum full > if the only open transaction is the one running the vacuum and it is a > clean transaction? It wouldn't eliminate more rows, but it could nonetheless produce a smaller table. IIRC, VACUUM FULL stops shrinking as soon as it finds a row that there is no room for in lower-numbered table pages; so a large row near the end of the table could block squeezing-out of small amounts of free space in earlier pages of the table. I doubt this effect is significant most of the time, but in a table with widely varying row sizes it might be an issue. Also, CLUSTER can definitely produce smaller *indexes* than VACUUM FULL. VACUUM FULL operates at a serious disadvantage when it comes to indexes, because in order to move a tuple it has to actually make extra index entries. >> I'm not at all sure I believe your premise that querying for a different >> key value excludes cache effects, btw. On modern hardware it's likely >> that CLUSTER would leave the *whole* of these tables sitting in kernel >> disk cache. >> > You are exactly right. After rebooting the entire box and running > the query the query time was 15 seconds. Rebooting the box, running > cluster on all three tables and then executing the query was 120 ms. Is > calling cluster the only way to ensure that these tables get loaded into > cache? Running select * appeared to cache some but not all. Hm, I'd think that SELECT * or SELECT count(*) would cause all of a table to be cached. It wouldn't do anything about caching the indexes though, and that might explain your observations. regards, tom lane
