SPI_getargtypeid — return the data type OID for an argument of
  a statement prepared by SPI_prepare
Oid SPI_getargtypeid(SPIPlanPtrplan, intargIndex)
   SPI_getargtypeid returns the OID representing the type
   for the argIndex'th argument of a statement prepared by
   SPI_prepare. First argument is at index zero.
  
SPIPlanPtr plan      prepared statement (returned by SPI_prepare)
     
int argIndexzero based index of the argument
    The type OID of the argument at the given index.
    If the plan is NULL or invalid,
    or argIndex is less than 0 or
    not less than the number of arguments declared for the
    plan,
    SPI_result is set to SPI_ERROR_ARGUMENT
    and InvalidOid is returned.