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.

1 files changed, 19 insertions, 1 deletions

diff --git a/django/db/backends/mysql/compiler.py b/django/db/backends/mysql/compiler.py
index fc74ac1991..96232aff51 100644
--- a/django/db/backends/mysql/compiler.py
+++ b/django/db/backends/mysql/compiler.py
@@ -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):