It would be nice if we also support three arguments substring for Ballista. substring impl: https://github.com/apache/arrow-datafusion/blob/master/datafusion-physical-expr/src/unicode_expressions.rs#L413 For DataFrame API: https://github.com/apache/arrow-datafusion/blob/master/datafusion-expr/src/expr_fn.rs#L256 For Ballista: https://github.com/apache/arrow-datafusion/blob/master/datafusion-proto/src/from_proto.rs#L1138-L1141