Re: ANALYZE to be ignored by VACUUM

"ITAGAKI Takahiro" <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> 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!