On Fri, Jul 29, 2005 at 11:54:18AM -0600, Lee Wu wrote: > explain select count(*) from pg_class where relname='pg_class'; > QUERY PLAN > -------------------------------------------------------------- > Aggregate (cost=4.55..4.55 rows=1 width=0) > -> Seq Scan on pg_class (cost=0.00..4.55 rows=1 width=0) > Filter: (relname = 'pg_class'::name) > (3 rows) > > Where and how PG get "rows=1"? The planner uses statistics to estimate the number of rows that a query will return. Here are a couple of links that provide more information: http://www.postgresql.org/docs/8.0/static/planner-stats.html http://www.postgresql.org/docs/8.0/static/view-pg-stats.html http://www.postgresql.org/docs/8.0/static/sql-analyze.html http://developer.postgresql.org/docs/postgres/planner-stats-details.html -- Michael Fuhr http://www.fuhr.org/~mfuhr/
