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Fixed #33025 -- Avoided accessing the database connections in Query.build_lookup() when not necessary. 

Of the built-in backends, only Oracle treats empty strings and nulls as
equal, so avoid testing the default connection backend for
interprets_empty_strings_as_nulls if it can be established from the
lookup that it wouldn't affect the lookup instance returned. This
improves performance a small amount for most lookups being built,
because accessing the connections requires touching the thread critical
`Local` which is an expensive operation.
This commit is contained in:
Keryn Knight 2021-08-14 15:30:54 +01:00 committed by Mariusz Felisiak
parent faf6d48590
commit bf5abf1bdc
1 changed files with 5 additions and 2 deletions
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7
django/db/models/sql/query.py
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@ -1192,8 +1192,11 @@ class Query(BaseExpression):
# stage because join promotion can't be done in the compiler. Using # stage because join promotion can't be done in the compiler. Using
# DEFAULT_DB_ALIAS isn't nice but it's the best that can be done here. # DEFAULT_DB_ALIAS isn't nice but it's the best that can be done here.
# A similar thing is done in is_nullable(), too. # A similar thing is done in is_nullable(), too.
if (connections[DEFAULT_DB_ALIAS].features.interprets_empty_strings_as_nulls and if (
lookup_name == 'exact' and lookup.rhs == ''): lookup_name == 'exact' and
lookup.rhs == '' and
connections[DEFAULT_DB_ALIAS].features.interprets_empty_strings_as_nulls
):
return lhs.get_lookup('isnull')(lhs, True) return lhs.get_lookup('isnull')(lhs, True)
return lookup return lookup