proposal: allow to specify result tupdesc and mode in SPI API

Hello

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

Comments, notes?

Regards

Pavel Stehule