Re: Questions on query planner, join types, and work_mem

Hannu Krosing <hannu@2ndquadrant.com> writes:
> Of course there are more variables than just *_page_cost, so if you nail
> down any other one, you may end with less than 1 for both page costs.

> I have always used seq_page_cost = 1 in my thinking and adjusted others
> relative to it.

Right, seq_page_cost = 1 is sort of the traditional reference point,
but you don't have to do it that way.  The main point here is that for
an all-in-RAM database, the standard page access costs are too high
relative to the CPU effort costs:

regression=# select name, setting from pg_settings where name like '%cost';
         name         | setting
----------------------+---------
 cpu_index_tuple_cost | 0.005
 cpu_operator_cost    | 0.0025
 cpu_tuple_cost       | 0.01
 random_page_cost     | 4
 seq_page_cost        | 1
(5 rows)

To model an all-in-RAM database, you can either dial down both
random_page_cost and seq_page_cost to 0.1 or so, or set random_page_cost
to 1 and increase all the CPU costs.  The former is less effort ;-)

It should be noted also that there's not all that much evidence backing
up the default values of the cpu_xxx_cost variables.  In the past those
didn't matter much because I/O costs always swamped CPU costs anyway.
But I can foresee us having to twiddle those defaults and maybe refine
the CPU cost model more, as all-in-RAM cases get more common.

            regards, tom lane