we have not possibility to simple specify result types in SPI API functions. Planner has this functionality - see transformInsertRow function, but it is not visible from SPI.
A new function should to look like:
SPIPlanPtr SPI_prepare_params_rettupdesc(const char *src, ParserSetupHook parserSetup, void *parserSetupArg, int cursorOptions,
TupDesc *retTupDesc,
int CoercionMode)
CoercionMode should be:
COERCION_MODE_SQL .. same as INSERT or UPDATE does
COERCION_MODE_SQL_NOERROR .. same as above with possible IO cast
COERCION_MODE_EXPLICIT .. same as using explicit casting
COERCION_MODE_EXPLICIT_NOERROR .. same as previous with possible IO cast
Benefits:
* simplify life to SPI users - no necessary late casting
* possible small simplification of plpgsql with two benefits:
** reduce performance impact of hidden IO cast
** reduce possible issues with type transformation via hidden IO cast
