#30079: Prefetch cache should be aware of database source and .using() should
not
always trigger a new query
-------------------------------------+-------------------------------------
Reporter: Mike Lissner | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: prefetch, | Triage Stage:
multidatabase | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Mike Lissner):
Replying to [comment:1 Carlton Gibson]:
> On the basis of the difference between `ln64` and `ln65` I'm inclined to
accept this. `pizza0` came from `replica` and the `ln64` call just goes
with that. (That seems right/expected to me...) I see your point that
adding the `using()` shouldn't change that.
Good! I made myself clear. Always a good thing.
> At first glance the ''"if you didn't add a `filter()`/`exclude()`"''
idea seems OK too.
That was my analysis too: "This seems to be a reasonable line in the
sand."
> Two questions though:
>
> 1. What's the use-case for re-adding the `using()` call in `ln65`? I
can't quite see why you'd do that...? (Another way of looking at `using()`
might be "go back to **this** DB"...)
I can't make a really clear argument for my use case, but basically it
was, "I'm doing weird things with prefetching and I want to be sure Django
pulls this data properly from the right DB/cache." In other words, it was
me trying to be explicit about where the caches were *supposed* come from,
and then getting bit by having the caches busted. From my reading, ln64 is
totally unclear which DB it would use since it would normally use the
"default" DB, but had a cached value, so which wins? My solution was to be
more explicit to make sure it used the cache, but that backfired (luckily,
I was watching the query logs).
> 2. Have you looked at all at what a patch might look like? (If it's
simple enough, that helps...)
No, not at all, unfortunately. I've looked into the Django code from time
to time, but mostly I stay on my side of the API, lest I get in trouble.
Especially when it comes to the ORM bits.
--
Ticket URL: <https://code.djangoproject.com/ticket/30079#comment:2>
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