Re: [DOCS] Please Help: PostgreSQL Query Optimizer

[ trimming cc list to something sane ]

"Anjan Kumar. A." <anjankumar@cse.iitb.ac.in> writes:
>     In Main Memory DataBase(MMDB) entire database on the disk is loaded  on to the main memory during initial startup
ofthe system.  There after all the references are made to database on the main memory.  When the system is going to
shutdown,we will write back the database on  the main memory to disk.  Here, for the sake of recovery we are writing
logrecords on to the disk  during the transaction execution.
 

Don't you get 99.9% of this for free with Postgres' normal behavior?
Just increase shared_buffers.

>   Can any one tell me the modifications needs to be incorporated to PostgreSQL,  so that it considers only Processing
Costsduring optimization of the Query.
 

Assuming that a page fetch costs zero is wrong even in an all-in-memory
environment.  So I don't see any reason you can't maintain the
convention that a page fetch costs 1.0 unit, and just adjust the other
cost parameters in the light of a different idea about what that
actually means.

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

You'd want random_page_cost = 1 since there is presumably no penalty for
random access in this context.  Also, I think you'd want
cpu_operator_cost a lot higher than that (maybe you dropped a decimal
place?  You scaled the others up by 200 but this one only by 20).

It's entirely possible that the ratios of the cpu_xxx_cost values
aren't very good and will need work.  In the past we've never had
occasion to study them very carefully, since they were only marginal
contributions anyway.
        regards, tom lane