SPI_returntuple makes a copy of a row in the upper executor context, returning it in the form of a row type Datum. The returned pointer need only be converted to Datum via
PointerGetDatum before returning.
Note that this should be used for functions that are declared to return composite types. It is not used for triggers; use
SPI_copytuple for returning a modified row in a trigger.
row to be copied
descriptor for row (pass the same descriptor each time for most effective caching)
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