Estimation of HashJoin Cost

    I need to estimate the hashjoin cost in my research. As the textbook shows, it is 3(R+S) where R and S are the size of the tablesize of the two tables, which realistically only considers the cost of IO. But this is obviously too theoretical. What is the correct way to estimate the cost of hashjoin? 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? 

    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?

Thanks

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