A Schema is a list of Fields, which map names to
Arrow data types. Create a Schema when you
want to convert an R data.frame to Arrow but don't want to rely on the
default mapping of R types to Arrow types, such as when you want to choose a
specific numeric precision, or when creating a Dataset and you want to
ensure a specific schema rather than inferring it from the various files.
Many Arrow objects, including Table and Dataset, have a $schema method
(active binding) that lets you access their schema.