"ITAGAKI Takahiro" <[EMAIL PROTECTED]> writes: > 4. ANALYZE finishes in a short time. > It is ok that VACUUM takes a long time because it is not a transaction, > but ANALYZE should not. It requres cleverer statistics algorithm. > Sampling factor 10 is not enough for pg_stats.n_distinct. We seems to > estimate n_distinct too low for clustered (ordered) tables.
Unfortunately no constant size sample is going to be enough for reliable n_distinct estimates. To estimate n_distinct you really have to see a percentage of the table, and to get good estimates that percentage has to be fairly large. There was a paper with a nice algorithm posted a while back which required only constant memory but it depended on scanning the entire table. I think to do n_distinct estimates we'll need some statistics which are either gathered opportunistically whenever a seqscan happens or maintained by an index. -- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's Slony Replication support! ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster
