[OSM-dev] Postgres 8.4/PostGIS 1.4 performance

Tom Hughes tom at compton.nu
Tue Sep 15 10:59:25 BST 2009


On 15/09/09 10:42, Frederik Ramm wrote:

> Now the two tables create more questions than they answer. Why does the
> node table under 8.4 seem to have less entries than under 8.3 when both
> have been populated with the same planet file?

An analyse without vacuum only generates an estimate for reltuples. See 
the doco at http://www.postgresql.org/docs/8.4/static/planner-stats.html 
which says:

For efficiency reasons, reltuples and relpages are not updated 
on-the-fly, and so they usually contain somewhat out-of-date values. 
They are updated by VACUUM, ANALYZE, and a few DDL commands such as 
CREATE INDEX. A stand-alone ANALYZE, that is one not part of VACUUM, 
generates an approximate reltuples value since it does not read every 
row of the table. The planner will scale the values it finds in pg_class 
to match the current physical table size, thus obtaining a closer 
approximation.

Tom

-- 
Tom Hughes (tom at compton.nu)
http://www.compton.nu/




More information about the dev mailing list