Qi Huang wrote:
> I need to estimate the hashjoin cost in my research.
> I looked at the code of final_cost_hashjoin() . It is not clear
> what factor it is considering. So, except the I/O to and from disk,
> what other factors are affecting the cost of hahsjoin?
http://www.postgresql.org/docs/9.2/interactive/runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS
If you don't find it practical to read the code, you could run
ANALYZE of a query which uses a hashjoin with different cost factors
(these can be changed for your current connection with the SET
command) and observe the cost numbers in the output.
> Also, is there any way to force postgres abide on the estimation of
> Hashjoin cost as 3(R+S), which also means, to make hashjoin cost
> mainly spend on I/O?
How useful would that be for workloads where data is fully cached?
-Kevin