Pergola rafter spans
A unloaded 19' long cedar 2x10 will sag about a little less than 0.2" which is L/>1000 and I would expect that to more than fit your needs.

HOWEVER: 19' long 2x10's will roll over in the middle meaning that they will twist in the middle with the appearance of the top tending to roll to one side as if attempting to roll completely over. The way to mitigate this is to add blocking. It is typical to add full depth blocking at 8' oc which would require two rows of blocking for your case.