On Sat, Mar 5, 2011 at 8:02 PM, Raymond O'Donnell <rod@xxxxxx> wrote: > On 03/03/2011 13:29, obamabarak@xxxxx wrote: >> >> I use pgsql 9.0.3 and I know that postgresql tries to use the fields in >> indexes instead of the original table if it possible >> >> But when I run >> >> SELECT COUNT(id) FROM tab >> >> or >> >> SELECT COUNT(*) FROM tab >> >> where there "id" is PRIMARY KEY and there are other indexes there I get >> execution plan that doesnt use any indexes, but sequentab scanning the >> original table. > > Because when you do SELECT COUNT(*) without any WHERE.... clause, then > PostgreSQL has to scan through *all* the rows in the table in order to count > them. > > Ray. > > -- > Raymond O'Donnell :: Galway :: Ireland > rod@xxxxxx > > -- > Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-general > Is it possible in theory to efficiently perform count the primary or unique indices underlying data structures, regardless whether there is a WHERE clause detailing filtration base on values from such index or not? Allan. -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
