Why could GEQO produce plans with lower costs than thestandard_join_search?

Hi,

I was expecting the plans generated by standard_join_search to have lower costs
than the plans from GEQO. But after the results I have from a join order
benchmark show that GEQO produces plans with lower costs most of the time!

I wonder what is causing this observation? From my understanding,
standard_join_search is doing a complete search. So I'm not sure how the GEQO
managed to do better than that.

Thank you,
Donald Dong