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Made some types of nested update queries very slightly more efficient…

… at the

database level. Also worked around the fact that MySQL (and maybe other
backends we don't know about) cannot select from the table they're updating.

Fixed #7095.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7496 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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commit ba010ec1c04c4832d42b088b092df0969afdb7e6 1 parent ff6e529
@malcolmt malcolmt authored
View
1  django/db/backends/__init__.py
@@ -51,6 +51,7 @@ class BaseDatabaseFeatures(object):
uses_case_insensitive_names = False
uses_custom_query_class = False
empty_fetchmany_value = []
+ update_can_self_select = True
class BaseDatabaseOperations(object):
"""
View
1  django/db/backends/mysql/base.py
@@ -63,6 +63,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
autoindexes_primary_keys = False
inline_fk_references = False
empty_fetchmany_value = ()
+ update_can_self_select = False
class DatabaseOperations(BaseDatabaseOperations):
def date_extract_sql(self, lookup_type, field_name):
View
1  django/db/backends/mysql_old/base.py
@@ -67,6 +67,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
autoindexes_primary_keys = False
inline_fk_references = False
empty_fetchmany_value = ()
+ update_can_self_select = False
class DatabaseOperations(BaseDatabaseOperations):
def date_extract_sql(self, lookup_type, field_name):
View
23 django/db/models/sql/subqueries.py
@@ -159,20 +159,37 @@ def pre_sql_setup(self):
# from other tables.
query = self.clone(klass=Query)
query.bump_prefix()
- query.select = []
query.extra_select = {}
- query.add_fields([query.model._meta.pk.name])
+ first_table = query.tables[0]
+ if query.alias_refcount[first_table] == 1:
+ # We can remove one table from the inner query.
+ query.unref_alias(first_table)
+ for i in xrange(1, len(query.tables)):
+ table = query.tables[i]
+ if query.alias_refcount[table]:
+ break
+ join_info = query.alias_map[table]
+ query.select = [(join_info[RHS_ALIAS], join_info[RHS_JOIN_COL])]
+ must_pre_select = False
+ else:
+ query.select = []
+ query.add_fields([query.model._meta.pk.name])
+ must_pre_select = not self.connection.features.update_can_self_select
# Now we adjust the current query: reset the where clause and get rid
# of all the tables we don't need (since they're in the sub-select).
self.where = self.where_class()
- if self.related_updates:
+ if self.related_updates or must_pre_select:
+ # Either we're using the idents in multiple update queries (so
+ # don't want them to change), or the db backend doesn't support
+ # selecting from the updating table (e.g. MySQL).
idents = []
for rows in query.execute_sql(MULTI):
idents.extend([r[0] for r in rows])
self.add_filter(('pk__in', idents))
self.related_ids = idents
else:
+ # The fast path. Filters and updates in one query.
self.add_filter(('pk__in', query))
for alias in self.tables[1:]:
self.alias_refcount[alias] = 0
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