SPI_gettypeid — return the data type OID of the specified column
Oid SPI_gettypeid(TupleDescrowdesc, intcolnumber)
   SPI_gettypeid returns the
   OID of the data type of the specified column.
  
TupleDesc rowdescinput row description
int colnumbercolumn number (count starts at 1)
   The OID of the data type of the specified column
   or InvalidOid on error.  On error,
   SPI_result is set to
   SPI_ERROR_NOATTRIBUTE.