Hello Hackers: I'm reading the code of optimizer and get confused about the 3 functions. add_path/set_cheapest/get_cheapest_fractional_path
add_(partial_)path: For every relations, optimizer will build path for it and add then call add_path to the rel->pathlist. during this stage, *it compare the current rel->pathlist with the new one, then it may discard it or add it into the pathlist. finally we may have multi path now. * *set_cheapest* after we finished the add_path for each relation, we will call set_cheapest, *the cheapest is the one with the lowest total cost plus the parameterized path. * *get_cheapest_fractional_path* *after we build the paths for all the relation, it calls *get_cheapest_fractional_path to get the best path and then build the plan. so my question is why do we need to have the 3 cost compare function? and i have another real case, and hope you can provide some hints: select * from t1 where slow_func(t1.a, 3) == true; ==> parallel bitmap index scan. select * from t1, t2 where t2.pk = 1 and t1.b = t2.b and slow_function(t1.a, t2.a) == true; ==> I get nest loop without parallel scan. what I hope is after I get the t2.a (only 1 value), I want to go with parallel index scan as well. In this case, which part should I focus on to figure out the issue? Thanks
