Hi, Dear Hackers    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?

Best RegardsHuang Qi VictorComputer Science of National University of Singapore 

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