$setWindowFields has the following fields:

• partitionBy
• sortBy
• output

"partitionBy" is an agg expression and should be serialized as one. "sortBy" is a sort expression and should be serialized accordingly. Within "output", window operators should be serialized as accumulator expressions. All other elements should be serialized as either enum types or constants. Note that strings representing enum types should be preserved whereas constant values should be abstracted during key generation.