From: Josh Berkus <josh@agliodbs.com>
Organization: Aglio Database Solutions
To: pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Please Help: PostgreSQL Query Optimizer
Date: Sun, 11 Dec 2005 11:26:18 -0800
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Cc: "Anjan Kumar. A." <anjankumar@cse.iitb.ac.in>, pgsql-chat@postgresql.org,
	pgsql-benchmarks@postgresql.org, pgsql-docs@postgresql.org
References: <Pine.LNX.4.61.0512111610320.4525@nsl-33.cse.iitb.ac.in>
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Anjan,

> In our case we are reading pages from Main Memory File System, but not from
> Disk. Will it be sufficient, if we change the  default values of above
> paramters in "src/include/optimizer/cost.h and 
> src/backend/utils/misc/postgresql.conf.sample" as follows:
>
>          random_page_cost = 4;

This should be dramatically lowered.  It's supposed to represent the ratio of 
seek-fetches to seq scans on disk.  Since there's no disk, it should be a 
flat 1.0.   However, we are aware that there are flaws in our calculations 
involving random_page_cost, such that the actual number for a system where 
there is no disk cost would be lower than 1.0.   Your research will hopefully 
help us find these flaws.

>          cpu_tuple_cost = 2;
>          cpu_index_tuple_cost = 0.2;
>          cpu_operator_cost = 0.05;

I don't see why you're increasing the various cpu_* costs.  CPU costs would be 
unaffected by the database being in memory.   In general, I lower these by a 
divisor based on the cpu speed; for example, on a dual-opteron system I lower 
the defaults by /6.   However, that's completely unrelated to using an MMDB.

So, other than random_page_cost, I don't know of other existing GUCs that 
would be directly related to using a disk/not using a disk.  How are you 
handling shared memory and work memory?

I look forward to hearing more about your test!

-- 
Josh Berkus
Aglio Database Solutions
San Francisco