The page said every Function output must be non-nullable. On this SDK
only a scalar output must be. A named-struct output may declare nullable
fields: whether a row has a result at all is the struct's own validity,
which Enterprise keeps in a separate internal column when it expands the
result into sibling columns.
test_function_output_nullability checks both rules against the SDK,
including that a struct declared as a single nullable field is refused.
Brendan Clement
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