2009/8/17 Jeremy Harris <jgh@wizmail.org>:
> Could not pgsql *measure* these costs (on a sampling basis, and with long
> time-constants)?
In theory, sure. In practice, well, there are some engineering
challenges to solve.
1) The cost model isn't perfect so the it's not clear exactly what to
measure to get the best result. As the cost model stands now you would
have to include some of the cpu time as well as the i/o time.
2) Timing is expensive. On many platforms getting the current time
takes a long time and would make a big difference to the runtime of
the query.
--
greg
http://mit.edu/~gsstark/resume.pdf