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Fixed #23576 -- Implemented multi-alias fast-path deletion in MySQL backend.

This required moving the entirety of DELETE SQL generation to the
compiler where it should have been in the first place and implementing
a specialized compiler on MySQL/MariaDB.

The MySQL compiler relies on the "DELETE table FROM table JOIN" syntax
for queries spanning over multiple tables.
This commit is contained in:
Simon Charette 2019-10-17 01:57:39 -04:00 committed by Mariusz Felisiak
parent e645f27907
commit 7acef095d7
5 changed files with 54 additions and 49 deletions

View File

@ -14,7 +14,25 @@ class SQLInsertCompiler(compiler.SQLInsertCompiler, SQLCompiler):
class SQLDeleteCompiler(compiler.SQLDeleteCompiler, SQLCompiler):
pass
def as_sql(self):
if self.single_alias:
return super().as_sql()
# MySQL and MariaDB < 10.3.2 doesn't support deletion with a subquery
# which is what the default implementation of SQLDeleteCompiler uses
# when multiple tables are involved. Use the MySQL/MariaDB specific
# DELETE table FROM table syntax instead to avoid performing the
# operation in two queries.
result = [
'DELETE %s FROM' % self.quote_name_unless_alias(
self.query.get_initial_alias()
)
]
from_sql, from_params = self.get_from_clause()
result.extend(from_sql)
where, params = self.compile(self.query.where)
if where:
result.append('WHERE %s' % where)
return ' '.join(result), tuple(from_params) + tuple(params)
class SQLUpdateCompiler(compiler.SQLUpdateCompiler, SQLCompiler):

View File

@ -744,7 +744,10 @@ class QuerySet:
Delete objects found from the given queryset in single direct SQL
query. No signals are sent and there is no protection for cascades.
"""
return sql.DeleteQuery(self.model).delete_qs(self, using)
query = self.query.clone()
query.__class__ = sql.DeleteQuery
cursor = query.get_compiler(using).execute_sql(CURSOR)
return cursor.rowcount if cursor else 0
_raw_delete.alters_data = True
def update(self, **kwargs):

View File

@ -7,13 +7,16 @@ from django.core.exceptions import EmptyResultSet, FieldError
from django.db.models.constants import LOOKUP_SEP
from django.db.models.expressions import OrderBy, Random, RawSQL, Ref, Value
from django.db.models.functions import Cast
from django.db.models.query_utils import QueryWrapper, select_related_descend
from django.db.models.query_utils import (
Q, QueryWrapper, select_related_descend,
)
from django.db.models.sql.constants import (
CURSOR, GET_ITERATOR_CHUNK_SIZE, MULTI, NO_RESULTS, ORDER_DIR, SINGLE,
)
from django.db.models.sql.query import Query, get_order_dir
from django.db.transaction import TransactionManagementError
from django.db.utils import DatabaseError, NotSupportedError
from django.utils.functional import cached_property
from django.utils.hashable import make_hashable
@ -1344,19 +1347,37 @@ class SQLInsertCompiler(SQLCompiler):
class SQLDeleteCompiler(SQLCompiler):
@cached_property
def single_alias(self):
return sum(self.query.alias_refcount[t] > 0 for t in self.query.alias_map) == 1
def _as_sql(self, query):
result = [
'DELETE FROM %s' % self.quote_name_unless_alias(query.base_table)
]
where, params = self.compile(query.where)
if where:
result.append('WHERE %s' % where)
return ' '.join(result), tuple(params)
def as_sql(self):
"""
Create the SQL for this query. Return the SQL string and list of
parameters.
"""
assert len([t for t in self.query.alias_map if self.query.alias_refcount[t] > 0]) == 1, \
"Can only delete from one table at a time."
qn = self.quote_name_unless_alias
result = ['DELETE FROM %s' % qn(self.query.base_table)]
where, params = self.compile(self.query.where)
if where:
result.append('WHERE %s' % where)
return ' '.join(result), tuple(params)
if self.single_alias:
return self._as_sql(self.query)
innerq = self.query.clone()
innerq.__class__ = Query
innerq.clear_select_clause()
pk = self.query.model._meta.pk
innerq.select = [
pk.get_col(self.query.get_initial_alias())
]
outerq = Query(self.query.model)
outerq.where = self.query.where_class()
outerq.add_q(Q(pk__in=innerq))
return self._as_sql(outerq)
class SQLUpdateCompiler(SQLCompiler):

View File

@ -3,7 +3,6 @@ Query subclasses which provide extra functionality beyond simple data retrieval.
"""
from django.core.exceptions import FieldError
from django.db import connections
from django.db.models.query_utils import Q
from django.db.models.sql.constants import (
CURSOR, GET_ITERATOR_CHUNK_SIZE, NO_RESULTS,
@ -41,40 +40,6 @@ class DeleteQuery(Query):
num_deleted += self.do_query(self.get_meta().db_table, self.where, using=using)
return num_deleted
def delete_qs(self, query, using):
"""
Delete the queryset in one SQL query (if possible). For simple queries
this is done by copying the query.query.where to self.query, for
complex queries by using subquery.
"""
innerq = query.query
# Make sure the inner query has at least one table in use.
innerq.get_initial_alias()
# The same for our new query.
self.get_initial_alias()
innerq_used_tables = tuple([t for t in innerq.alias_map if innerq.alias_refcount[t]])
if not innerq_used_tables or innerq_used_tables == tuple(self.alias_map):
# There is only the base table in use in the query.
self.where = innerq.where
else:
pk = query.model._meta.pk
if not connections[using].features.update_can_self_select:
# We can't do the delete using subquery.
values = list(query.values_list('pk', flat=True))
if not values:
return 0
return self.delete_batch(values, using)
else:
innerq.clear_select_clause()
innerq.select = [
pk.get_col(self.get_initial_alias())
]
values = innerq
self.where = self.where_class()
self.add_q(Q(pk__in=values))
cursor = self.get_compiler(using).execute_sql(CURSOR)
return cursor.rowcount if cursor else 0
class UpdateQuery(Query):
"""An UPDATE SQL query."""

View File

@ -535,9 +535,7 @@ class FastDeleteTests(TestCase):
a = Avatar.objects.create(desc='a')
User.objects.create(avatar=a)
u2 = User.objects.create()
expected_queries = 1 if connection.features.update_can_self_select else 2
self.assertNumQueries(expected_queries,
User.objects.filter(avatar__desc='a').delete)
self.assertNumQueries(1, User.objects.filter(avatar__desc='a').delete)
self.assertEqual(User.objects.count(), 1)
self.assertTrue(User.objects.filter(pk=u2.pk).exists())