Re: [HACKERS] Please Help: PostgreSQL Query Optimizer

Since sequential access is not significantly faster than random access in a MMDB, random_page_cost will be approximately same as sequential page fetch cost.

As every thing is present in Main Memory, we need to give approximately same cost to read/write to Main Memory and CPU Related operations.

But, in PostgreSQL all costs are scaled relative to a page fetch. If we make both sequential_page_fetch_cost and random_page_cost to "1", then we need to increase the various cpu_* paramters by multiplying the default values with appropriate Scaling Factor. Now, we need to determine this Scaling Factor.

Still, i want to confirm whether this approach is the correct one.

On Sun, 11 Dec 2005, Josh Berkus wrote:

> 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!
>
>

--
Regards.

Anjan Kumar A.
MTech2, Comp Sci.,
www.cse.iitb.ac.in/~anjankumar
______________________________________________________________
Do not handicap your children by making their lives easy.
-- Robert Heinlein