SPI_getargtypeid returns the OID
representing the type for the argIndex'th argument of a plan prepared by
SPI_prepare. First argument is at
execution plan (returned by
zero 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.