values_list.py

# takes more time because it loads the whole model object into Python memory
%timeit upcs = [product.upc for product in Product.objects.all()]                                                                                                                                                                                    
3.91 ms ± 131 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

# faster than the above code because an SQL expression executes this
%timeit Product.objects.values_list('upc', flat=True)                                                                                                                                                                                                           
32.3 µs ± 1.12 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)