[petergeoghegan]

That may be true, but that doesn't seem like the important takeaway to me. The important takeaway is "In contrast to cardinality estimation, the contribution of the cost model to the overall query performance is limited". Actually, the paper itself says "This improvement [the 34% one you mention] is not insignificant, but on the other hand, it is dwarfed by improvement in query runtime observed when we replace estimated cardinalities with the real ones".

Optimizers are weird.

[jzoch]

Oh definitely agree. Mainly I find the cost model interesting because it’s so simple and contained. Cardinality estimation is a hard problem and requires real expertise. But the easy wins you get by just throwing out something based on old assumptions like the cost model is fun!