On 10/10/2007, James Shaw <[EMAIL PROTECTED]> wrote: > > On 10/10/2007, Tom Lane <[EMAIL PROTECTED]> wrote: > > "James Shaw" <[EMAIL PROTECTED]> writes: > > > The example given has "Seq Scan on tenk1 (cost=0.00..458.00" in the > example, > > > but then says, "you will find out that tenk1 has 358 disk pages" and > "the > > > cost is estimated at 358 page reads". Shouldn't this be 458 disk page > > > reads? > > > > No. Why did you stop reading in mid-sentence? > > I'm sorry, I misunderstood. I've reread, and understand where the > extra 100 comes from now. Perhaps this could be reworded: > > This is about as straightforward as it gets. If you do > SELECT relpages, reltuples FROM pg_class WHERE relname = 'tenk1'; > > you will find out that tenk1 has 358 disk pages and 10000 rows. The > estimated cost is (disk pages read * seq_page_cost) + (number of rows > read * cpu_tuple_cost). By default, seq_page_cost is 1.0 and > cpu_tuple_cost is 0.01. Therefore, the estimated cost is (358 * 1.0) > + (10000 * 0.01) = 458.
Were there any thoughts on this? I was hoping this might be included in the manual, since I felt it was somewhat clearer. James
