optimization in C

This solution isn't good when there are +10000 tuples in the table, it's
slowly...
anybody can help me ? :

string = "SELECT service, noeud, rubrique FROM table" ;
res = PQexec( conn, string.data() ) ;
if ( (! res) || (status = PQresultStatus( res ) != PGRES_TUPLES_OK) )
{
cerr << _ERROR << "Problem SELECT ! " << endl ;
cerr << _ERROR << "Error : " << PQresStatus( status ) << endl ;
cerr << _ERROR << "Error : " << PQresultErrorMessage( res ) << endl ;
PQclear( res ) ;
}
else
{
for (int m=0; m < PQntuples( res ); m++)
{
service = PQgetvalue( resultat1, m, 0 ) ;
noeud = PQgetvalue( resultat1, m, 1 ) ;
rubrique = PQgetvalue( resultat1, m, 2 ) ;

commande = "SELECT SUM(date) FROM table WHERE service='" + service +
"' AND noeud='" + noeud + "' AND rubrique='"+ rubrique + "'" ;
res1 = PQexec( conn, string.data() ) ;
if ( (! res1) || (status = PQresultStatus( res1 ) != PGRES_TUPLES_OK)
)
{
cerr << _ERROR << "Problem SUM ! " << endl ;
cerr << _ERROR << "Error : " << PQresStatus( status ) << endl ;
cerr << _ERROR << "Error : " << PQresultErrorMessage( res1 ) << endl
;
PQclear( res1 ) ;
}
else
{
cout << _TRACE << "SUM ok." << endl ;
PQclear( res1 ) ;
}
}
PQclear( res ) ;
}

Thanks. jerome.