Is there no longer a need for num_rows()?

I think you're looking for $builder->countAllResults()

Yeah, but that queries the database again, which num_rows() in CI3 doesn't do. I guess I'll do the count() once and then keep the result as a variable.

Digging deeper I see that BaseResult has a public property - $numRows. It appears that by using any db operation that returns a Result you should be able to do something like this.

PHP Code:

$res = $db->query("YOUR QUERY")->getResult();
$count = $res->numRows; 


I haven't confirmed my premise, only explored the source code. However, after exploring the source more deeply I'm skeptical about my correctness. I cannot find any place where $numRows is assigned a value. (I"m looking at the current state of the develop branch - last commit on Aug 11, 2019)

Even if I'm wrong that property might be a useful place to store the result of your count() call.