Which aspects should I consider when deciding whether to use simple function or vector function to implement Scalar function in Velox?

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Hello folks. I'm working on implementing our custom functions in Velox. This document shows two way to implement a Scalar function using either the simple function API or as a vector function. I would like to know which aspects should I consider when deciding whether to use simple function or vector function to implement Scalar function in Velox? Appreciate for any reply.