Re: Script to compute random page cost

On Mon, 2002-09-09 at 07:13, Bruce Momjian wrote:
> 
> OK, turns out that the loop for sequential scan ran fewer times and was
> skewing the numbers.  I have a new version at:
> 
>     ftp://candle.pha.pa.us/pub/postgresql/randcost

Latest version:

olly@linda$ 
random test:       14
sequential test:   11
null timing test:  9
random_page_cost = 2.500000

olly@linda$ for a in 1 2 3 4 5
> do
> ~/randcost
> done
Collecting sizing information ...
random test:       11
sequential test:   11
null timing test:  9
random_page_cost = 1.000000

random test:       11
sequential test:   10
null timing test:  9
random_page_cost = 2.000000

random test:       11
sequential test:   11
null timing test:  9
random_page_cost = 1.000000

random test:       11
sequential test:   10
null timing test:  9
random_page_cost = 2.000000

random test:       10
sequential test:   10
null timing test:  10
Sequential time equals null time.  Increase TESTCYCLES and rerun.


Available memory (512M) exceeds the total database size, so sequential
and random are almost the same for the second and subsequent runs.
Since, in production, I would hope to have all active tables permanently
in RAM, would there be a case for my using a page cost of 1 on the
assumption that no disk reads would be needed?

-- 
Oliver Elphick                                Oliver.Elphick@lfix.co.uk
Isle of Wight, UK                            
http://www.lfix.co.uk/oliver
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