I agree with your reasoning, and also favour option 2 now, especially since
the default can break on sqlite.

It would also be possible to go through a deprecation period to first raise
a warning and not prefetch, before a later version raises an exception,
which is probably kinder since previously it was documented that it just
was a no-op.

On Mon, 14 Jan 2019 at 19:03, charettes <charett...@gmail.com> wrote:

> > This seems reasonable, but would you do in the case chunk_size isn't
> explicitly defined - throw an exception?
>
> That's would be the plan.
>
> > Currently it silently fails to prefetch which means N + 1 queries, so
> even prefetching for the default chunk_size of 2000 would be a huge gain in
> cases where chunk_size isn't defined.
>
> That's assuming related relationships are actually accessed during
> iteration over the objects. If it's not currently the case turning that on
> would cause P + C extra queries.
>
> To summarize here's the options we have:
>
> 1. Always turn on the prefetching feature.
> 2. Require chunk_size to be explicitly specified for a deprecation period
> in order to enable the feature.
>
> Pros/Cons of 1. is that developers who actually didn't follow the
> documentation and/or didn't assert that prefetching didn't work and left
> some lingering prefetch_related() will probably see their code perform less
> queries but result in a slight increase in memory (given they discard
> objects on iterating). This is also likely to break code because of some
> backends (e.g. SQLite) query parameters limits (chunk_size=2000 > 1000)[0]
>
> Pros/Cons of 2. is that most prefetch_related().iterator() won't break for
> developers that followed the documentation and fail for others requiring
> them to acknowledge what their code is going to start doing.
>
> As I've expressed in my previous messages I'm slightly in favor of 2 even
> if it is likely to cause more breakage because of the exceptional situation
> where this API should have raised an exception from the beginning.
>
> Cheers,
> Simon
>
> [0] https://code.djangoproject.com/ticket/27833
>
> Le lundi 14 janvier 2019 09:53:45 UTC-5, Adam Johnson a écrit :
>>
>> ...what if we required chunk_size to be explicitly specified when the
>>> queryset has prefetch lookups?
>>
>>
>> This seems reasonable, but would you do in the case chunk_size isn't
>> explicitly defined - throw an exception? Currently it silently fails to
>> prefetch which means N + 1 queries, so even prefetching for the default
>> chunk_size of 2000 would be a huge gain in cases where chunk_size isn't
>> defined.
>>
>> On Sun, 13 Jan 2019 at 02:05, charettes <chare...@gmail.com> wrote:
>>
>>> Replying to my concerns about the P * C queries.
>>>
>>> Throwing that idea out there but what if we required chunk_size to be
>>> explicitly specified when the queryset has prefetch lookups?
>>>
>>> That would at least force the developers to consider how many
>>> prefetching queries iterating through the results would require. Plus since
>>> the argument was only recently introduced it is less likely to silently
>>> cause changes under developers feet.
>>>
>>> Simon
>>>
>>> Le vendredi 26 octobre 2018 20:12:02 UTC-4, charettes a écrit :
>>>>
>>>> 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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>>
>> --
>> Adam
>>
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