not sure about constraints

Hello,

I've got 4 tables :

CREATE TABLE tableA (id SERIAL PRIMARY KEY NOT NULL,                    blahA TEXT);                    
CREATE TABLE tableB (id SERIAL PRIMARY KEY NOT NULL,                    blahB TEXT);                    
CREATE TABLE tableC (id SERIAL PRIMARY KEY NOT NULL,                                         ida INT4 REFERENCES
tableA(id),                   idb INT4 REFERENCES tableB(id),                    blahC TEXT);                    
 
CREATE TABLE tableD    (id SERIAL PRIMARY KEY NOT NULL,                         ida INT4,    idb INT4,    blahC TEXT,
CONSTRAINT checkAB FOREIGN KEY (ida, idb) REFERENCES tableC(ida, idb));                    
 
Is the definition of tableD sufficient, or should I do it this way 
instead : 

CREATE TABLE tableD    (id SERIAL PRIMARY KEY NOT NULL,                         ida INT4 REFERENCES tableA(id),    idb
INT4REFERENCES tableB(id),    blahD TEXT,    CONSTRAINT checkAB FOREIGN KEY (ida, idb) REFERENCES tableC(ida, idb));
               
 
which looks superfluous to me.                     

???

Thanks in advance

Jerome Alet