Github user wzhfy commented on the issue:
https://github.com/apache/spark/pull/17240
@hvanhovell why simplifying the cost function:
1. Simplify cost by removing size, then we don't have to care about column
pruning while building new join nodes, because otherwise columm pruning will
influence the size of intermediate output and thus the cost of plan. This makes
the code much conciser. And I think ordering join first and then pruning
columns is easier and more natural.
2. Cardinality is often much more important than size in join reorder. BTW,
Postgresql also uses cardinality (and physical costs) in join reorder.
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