The idea of converting a JSONB array to a PG array is appealing and would potentially be more general-purpose than adding a new unnest. I'm not sure how feasible either suggestion is.
The one part I think is missing right now is unnest allows you to 'stitch' or 'zip' multiple arrays together into a single recordset via unnest(array1, array2, ...). Presumably that could be added to the json equivalents.
You can just use "ROWS FROM" to get the same result.
The special table function UNNEST may be called with any number of array parameters, and it returns a corresponding number of columns, as if UNNEST (Section 9.18) had been called on each parameter separately and combined using the ROWS FROM construct.
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Yes, the unnest form can be used within the target-list but that argument is not going to get you very far as that use of SRF is greatly frowned upon now that we have LATERAL.