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[3.0.x] Refs #26207 -- Removed obsolete note about slow constructing a model with deferred fields.

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This is not true since 7f51876 removed the necessity of creating
proxy model classes at runtime for each deferred field sets.
Backport of 35396a7f243eceec42cc90725ab573a7d9ac3b4c from master
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Simon Charette 2019-10-14 06:51:07 -04:00 committed by Mariusz Felisiak
parent e17e19cfa4
commit 8d196e6fea
1 changed files with 6 additions and 8 deletions
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14
docs/topics/db/optimization.txt
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@ -233,14 +233,12 @@ you know that you won't need (or won't need in most cases) to avoid loading
them. Note that if you *do* use them, the ORM will have to go and get them in
a separate query, making this a pessimization if you use it inappropriately.
Also, be aware that there is some (small extra) overhead incurred inside
Django when constructing a model with deferred fields. Don't be too aggressive
in deferring fields without profiling as the database has to read most of the
non-text, non-VARCHAR data from the disk for a single row in the results, even
if it ends up only using a few columns. The ``defer()`` and ``only()`` methods
are most useful when you can avoid loading a lot of text data or for fields
that might take a lot of processing to convert back to Python. As always,
profile first, then optimize.
Don't be too aggressive in deferring fields without profiling as the database
has to read most of the non-text, non-VARCHAR data from the disk for a single
row in the results, even if it ends up only using a few columns. The
``defer()`` and ``only()`` methods are most useful when you can avoid loading a
lot of text data or for fields that might take a lot of processing to convert
back to Python. As always, profile first, then optimize.
Use ``QuerySet.count()``
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