From 89032876f427a77ab4de26493190280377567d1c Mon Sep 17 00:00:00 2001
From: c-bata <contact@c-bata.link>
Date: Fri, 27 Mar 2020 01:51:30 +0900
Subject: [PATCH] Fixed #31275 -- Optimized sql_flush() without resetting
 sequences on MySQL.

Co-Authored-By: Simon Charette <charettes@users.noreply.github.com>
---
 AUTHORS                                |  1 +
 django/db/backends/mysql/operations.py | 47 ++++++++++++++++++--------
 docs/releases/3.1.txt                  | 10 ++++++
 3 files changed, 44 insertions(+), 14 deletions(-)

diff --git a/AUTHORS b/AUTHORS
index 371bfe070cc..d0b01a5eb2e 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -585,6 +585,7 @@ answer newbie questions, and generally made Django that much better:
     Martin von Gagern <gagern@google.com>
     Mart Sõmermaa <http://mrts.pri.ee/>
     Marty Alchin <gulopine@gamemusic.org>
+    Masashi Shibata <m.shibata1020@gmail.com>
     masonsimon+django@gmail.com
     Massimiliano Ravelli <massimiliano.ravelli@gmail.com>
     Massimo Scamarcia <massimo.scamarcia@gmail.com>
diff --git a/django/db/backends/mysql/operations.py b/django/db/backends/mysql/operations.py
index a8e9af2263e..9d69ba11523 100644
--- a/django/db/backends/mysql/operations.py
+++ b/django/db/backends/mysql/operations.py
@@ -194,21 +194,40 @@ class DatabaseOperations(BaseDatabaseOperations):
         return 'RETURNING %s' % ', '.join(columns), ()
 
     def sql_flush(self, style, tables, sequences, allow_cascade=False):
-        # NB: The generated SQL below is specific to MySQL
-        # 'TRUNCATE x;', 'TRUNCATE y;', 'TRUNCATE z;'... style SQL statements
-        # to clear all tables of all data
-        if tables:
-            sql = ['SET FOREIGN_KEY_CHECKS = 0;']
-            for table in tables:
-                sql.append('%s %s;' % (
-                    style.SQL_KEYWORD('TRUNCATE'),
-                    style.SQL_FIELD(self.quote_name(table)),
-                ))
-            sql.append('SET FOREIGN_KEY_CHECKS = 1;')
-            sql.extend(self.sequence_reset_by_name_sql(style, sequences))
-            return sql
-        else:
+        if not tables:
             return []
+        sql = ['SET FOREIGN_KEY_CHECKS = 0;']
+        tables = set(tables)
+        with_sequences = set(s['table'] for s in sequences)
+        # It's faster to TRUNCATE tables that require a sequence reset since
+        # ALTER TABLE AUTO_INCREMENT is slower than TRUNCATE.
+        sql.extend(
+            '%s %s;' % (
+                style.SQL_KEYWORD('TRUNCATE'),
+                style.SQL_FIELD(self.quote_name(table_name)),
+            ) for table_name in tables.intersection(with_sequences)
+        )
+        # Otherwise issue a simple DELETE since it's faster than TRUNCATE
+        # and preserves sequences.
+        sql.extend(
+            '%s %s %s;' % (
+                style.SQL_KEYWORD('DELETE'),
+                style.SQL_KEYWORD('FROM'),
+                style.SQL_FIELD(self.quote_name(table_name)),
+            ) for table_name in tables.difference(with_sequences)
+        )
+        sql.append('SET FOREIGN_KEY_CHECKS = 1;')
+        return sql
+
+    def sequence_reset_by_name_sql(self, style, sequences):
+        return [
+            '%s %s %s %s = 1;' % (
+                style.SQL_KEYWORD('ALTER'),
+                style.SQL_KEYWORD('TABLE'),
+                style.SQL_FIELD(self.quote_name(sequence_info['table'])),
+                style.SQL_FIELD('AUTO_INCREMENT'),
+            ) for sequence_info in sequences
+        ]
 
     def validate_autopk_value(self, value):
         # MySQLism: zero in AUTO_INCREMENT field does not work. Refs #17653.
diff --git a/docs/releases/3.1.txt b/docs/releases/3.1.txt
index 492622d3bd8..70241b1809a 100644
--- a/docs/releases/3.1.txt
+++ b/docs/releases/3.1.txt
@@ -347,6 +347,10 @@ Models
 * :meth:`.QuerySet.bulk_create` now sets the primary key on objects when using
   MariaDB 10.5+.
 
+* The ``DatabaseOperations.sql_flush()`` method now generates more efficient
+  SQL on MySQL by using ``DELETE`` instead of ``TRUNCATE`` statements for
+  tables which don't require resetting sequences.
+
 Pagination
 ~~~~~~~~~~
 
@@ -415,6 +419,12 @@ Tests
 * :class:`~django.test.runner.DiscoverRunner` now skips running the system
   checks on databases not :ref:`referenced by tests<testing-multi-db>`.
 
+* :class:`~django.test.TransactionTestCase` teardown is now faster on MySQL
+  due to :djadmin:`flush` command improvements. As a side effect the latter
+  doesn't automatically reset sequences on teardown anymore. Enable
+  :attr:`.TransactionTestCase.reset_sequences` if your tests require this
+  feature.
+
 URLs
 ~~~~