Re: Script to compute random page cost

OK, what you are seeing here is that for your platform the TESTCYCLES
size isn't enough;  the numbers are too close to measure the difference.

I am going to increase the TESTCYCLES from 5k to 10k.  That should
provide better numbers.

---------------------------------------------------------------------------

Oliver Elphick wrote:
> 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
> GPG: 1024D/3E1D0C1C: CA12 09E0 E8D5 8870 5839  932A 614D 4C34 3E1D 0C1C
>                  ========================================
>      "Draw near to God and he will draw near to you.  
>       Cleanse your hands, you sinners; and purify your  
>       hearts, you double minded."       James 4:8 
> 
> 

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