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 INT4 REFERENCES tableB(id),
blahD TEXT,
CONSTRAINT checkAB FOREIGN KEY (ida, idb) REFERENCES tableC(ida, idb));

which looks superfluous to me.

???

Thanks in advance

Jerome Alet