Should I iterate on django query set or over the variable?

Question

I have a large dataset returned from django queryset and I want to iterate over it. Should I directly iterate over the queryset or store the results in a variable and iterate over it?

for item in Model.objects.all():
    do_something()

or

results = Model.objects.all():
for item in results:
    do_something()

As far as I know, the variables are stored in heap and its safer, where as in case of iterating over queryset, the results will be stored in main memory.

So which one is efficient in space and speed?

both are same, but if you use a variable it just one more line of code, when you do all it saves it in the memory and 2nd time it doesnt query the database. so you can understand, — Exprator, May 19 '17 at 09:17

score 18 · Accepted Answer · edited May 23 '17 at 12:34

There is no difference. Python does not distinguish between data on the heap and "main memory" (or the stack); in CPython at least, all data is stored on the heap, and the stack contains references to that data. See this question.

The only consideration here is whether you need to refer to the queryset again in the same scope. If you do, store it in a variable; if not, then there is no need to.

score 9 · Answer 2 · answered May 19 '17 at 09:21

If your dataset is huge you could use iterator to reduce memory load and improve performance, e.g.

results = Model.objects.all()

for item in results.iterator():
    do_something()

You should only do this if it's really necessary, as it disables queryset caching. Re-using the queryset after this will result in poorer performance.

Should I iterate on django query set or over the variable?

2 Answers2