is there a way to optimize spark sql code?
Both behaviors are more or less expected. Spark is rather lazy and it not only doesn't execute transformations unless you trigger an action but can also skip tasks if there are not required for the output. Since first requires only a single element it can compute only one partition. It is most likely the reason why you see only one running thread at some point.
Regarding the second issue it is most likely a matter of configuration. Assuming there is nothing wrong with YARN configuration (I don't use YARN but yarn.nodemanager.resource.cpu-vcores looks like a possible source of the problem) it is most likely a matter of Spark defaults. As you can read in the Configuration guide spark.executor.cores on Yarn is by default set to 1. Two workers gives two running threads.