SPI_register_relation — make an ephemeral named relation available by name in SPI queries
int SPI_register_relation(EphemeralNamedRelation enr)   SPI_register_relation makes an ephemeral named
   relation, with associated information, available to queries planned and
   executed through the current SPI connection.
  
EphemeralNamedRelation enrthe ephemeral named relation registry entry
If the execution of the command was successful then the following (nonnegative) value will be returned:
SPI_OK_REL_REGISTERif the relation has been successfully registered by name
On error, one of the following negative values is returned:
SPI_ERROR_ARGUMENT       if enr is NULL or its
       name field is NULL
      
SPI_ERROR_UNCONNECTEDif called from an unconnected procedure
SPI_ERROR_REL_DUPLICATE       if the name specified in the name field of
       enr is already registered for this connection