https://code.djangoproject.com/ticket/29984 suggests to support 
prefetch_related() with QuerySet.iterator(). I accepted the ticket to do 
something and linked back to this discussion.

On Friday, October 26, 2018 at 8:12:02 PM UTC-4, charettes wrote:
>
> Josh, I agree that silently not working is problematic but it has been 
> this way since prefetch_related() was introduced.
>
> Something to keep in mind as well is that silently turning it on would 
> also perform P * C extra queries where P is the number of prefetches 
> requested through prefetch_related() and C the number of chunks the results 
> contains. This is non negligible IMO.
>
> What I'd be in favor off is raising an exception on 
> prefetch_related(*prefetches).iterator() in the next release at least to 
> allow users using this pattern to adjust their code and then ship the 
> optimization with all the warnings related to the interactions between 
> prefetch_related(*prefetches) and iterator(chunk_size) in the next one.
>
> I'm not completely completely against skipping the exception release phase 
> entirely given there might be users out there accessing attributes expected 
> to be prefetched on iterated instances and inadvertently performing tons of 
> queries but the exception phase just feels safer given iterator() is 
> usually used in memory constrained contexts.
>
> Simon
>
> Le vendredi 26 octobre 2018 19:27:55 UTC-4, Josh Smeaton a écrit :
>>
>> I tend to agree with Tobi. Prefetching silently not working on iterator 
>> can be quite confusing, unless you have a good understanding of both APIs. 
>> It might be possible to do what you're asking, but it'd mean that django is 
>> now actually caching the result when it explicitly says it isn't - even if 
>> the result is a much smaller moving cache. Prefetching chunk_size results 
>> per chunk is unlikely to make a material difference to memory usage. Users 
>> are usually concerned about the entire result set of the primary table.
>>
>> I don't know if you can change the API to make these suggested changes 
>> without also impacting how we iterate over result sets - but I'd be 
>> interested in seeing a proof of concept at the very least.
>>
>>
>>
>> On Monday, 15 October 2018 20:41:13 UTC+11, tobias....@truffls.com wrote:
>>>
>>> Thank you for your feedback. I would like to answer some statements to 
>>> either convince you or make it more clear, where my idea stems from:
>>>
>>> The fundamental reason why iterator() cannot be used with 
>>>> prefetch_related() is that the latter requires a set of model instance to 
>>>> be materialized to work appropriately which chunk_size doesn't control at 
>>>> all.
>>>> In other words chunk_size only controls how many rows should be fetched 
>>>> from the database cursor and kept into memory at a time. Even when this 
>>>> parameter is used, iterator() will only materialize a single model 
>>>> instance 
>>>> per yield.
>>>>
>>>  
>>> It should be easily possible to change the involved code of 
>>> ModelIterable to materialize the retrieved rows in batches. After 
>>> materializing the batch / chunk, it could do the prefetching.
>>>  
>>>
>>>> Given that iterator() always ignored prefetch related lookups instead 
>>>> of erroring out when they were specified make me feel like turning such a 
>>>> feature on by default could be problematic as it could balloon the memory 
>>>> usage which is the main reason why iterator is useful anyway.
>>>>
>>>
>>> I would argue, that users who thoughtlessly applied prefetching together 
>>> with iterator now actually get, what they thought of: less DB query round 
>>> trips traded against a little more memory usage.
>>>
>>> Best,
>>> Tobi
>>>
>>

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