UPDATE ... WHERE ... ORDER BY... always does a filesort when not required

This bug is originally reported for PXC version PXC 5.5.34.

This is easy to confirm with PS 5.6.17:

openxs@ao756:~/dbs/p5.6$ mysql -uroot -proot test
Warning: Using a password on the command line interface can be insecure.
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 1
Server version: 5.6.17-65.0-587.saucy (Ubuntu)

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owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> CREATE TABLE update_test (a INT AUTO_INCREMENT PRIMARY KEY, b INT, c INT,
    -> INDEX (b,c));
Query OK, 0 rows affected (0,93 sec)

mysql> INSERT INTO update_test (b, c) VALUES (1, 1), (1, 2), (1,3), (2, 2), (2,
    -> 3), (2,4), (2, 5), (5, 5), (6,6), (7,7);
Query OK, 10 rows affected (0,08 sec)
Records: 10 Duplicates: 0 Warnings: 0

mysql> SELECT * FROM update_test WHERE b = 1 ORDER BY c LIMIT 2;
+---+------+------+
| a | b | c |
+---+------+------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
+---+------+------+
2 rows in set (0,06 sec)

mysql> SHOW SESSION STATUS LIKE 'Sort%';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Sort_merge_passes | 0 |
| Sort_range | 0 |
| Sort_rows | 0 |
| Sort_scan | 0 |
+-------------------+-------+
4 rows in set (0,00 sec)

mysql> explain SELECT * FROM update_test WHERE b = 1 ORDER BY c LIMIT 2;
+----+-------------+-------------+------+---------------+------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+------+---------------+------+---------+-------+------+--------------------------+
| 1 | SIMPLE | update_test | ref | b | b | 5 | const | 3 | Using where; Using index |
+----+-------------+-------------+------+---------------+------+---------+-------+------+--------------------------+
1 row in set (0,02 sec)

mysql> UPDATE update_test SET a = a + 10 WHERE b = 1 ORDER BY c LIMIT 2;
Query OK, 2 rows affected (0,07 sec)
Rows matched: 2 Changed: 2 Warnings: 0

mysql> SHOW SESSION STATUS LIKE 'Sort%';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Sort_merge_passes | 0 |
| Sort_range | 1 |
| Sort_rows | 2 |
| Sort_scan | 0 |
+-------------------+-------+
4 rows in set (0,00 sec)

mysql> explain UPDATE update_test SET a = a + 10 WHERE b = 1 ORDER BY c LIMIT 2;
+----+-------------+-------------+-------+---------------+------+---------+-------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+------+---------+-------+--...

Confirmed on 5.6.15-56-log Percona XtraDB Cluster

mysql> show global variables like '%version%';
+-------------------------+----------------------------------------------------------------------------+
| Variable_name | Value |
+-------------------------+----------------------------------------------------------------------------+
| innodb_version | 5.6.15-rel63.0 |
| protocol_version | 10 |
| slave_type_conversions | |
| version | 5.6.15-56-log |
| version_comment | Percona XtraDB Cluster (GPL), Release 25.5, Revision 759, wsrep_25.5.r4061 |
| version_compile_machine | x86_64 |
| version_compile_os | Linux |
+-------------------------+----------------------------------------------------------------------------+

mysql> use test
Database changed

mysql> CREATE TABLE update_test (a INT AUTO_INCREMENT PRIMARY KEY, b INT, c INT, INDEX (b,c));
Query OK, 0 rows affected (0.16 sec)

mysql> INSERT INTO update_test (b, c) VALUES (1, 1), (1, 2), (1,3), (2, 2), (2,3), (2,4), (2, 5), (5, 5), (6,6), (7,7);
Query OK, 10 rows affected (0.01 sec)
Records: 10 Duplicates: 0 Warnings: 0

mysql> SELECT * FROM update_test WHERE b = 1 ORDER BY c LIMIT 2;
+---+------+------+
| a | b | c |
+---+------+------+
| 2 | 1 | 1 |
| 4 | 1 | 2 |
+---+------+------+
2 rows in set (0.03 sec)

mysql> SHOW SESSION STATUS LIKE 'Sort%';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Sort_merge_passes | 0 |
| Sort_range | 0 |
| Sort_rows | 0 |
| Sort_scan | 0 |
+-------------------+-------+
4 rows in set (0.00 sec)

mysql> explain SELECT * FROM update_test WHERE b = 1 ORDER BY c LIMIT 2;
+----+-------------+-------------+------+---------------+------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+------+---------------+------+---------+-------+------+--------------------------+
| 1 | SIMPLE | update_test | ref | b | b | 5 | const | 3 | Using where; Using index |
+----+-------------+-------------+------+---------------+------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)

mysql> UPDATE update_test SET a = a + 13 WHERE b = 1 ORDER BY c LIMIT 2;
Query OK, 2 rows affected (0.69 sec)
Rows matched: 2 Changed: 2 Warnings: 0

mysql> SHOW SESSION STATUS LIKE 'Sort%';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Sort_merge_passes | 0 |
| Sort_range | 1 |
| Sort_rows | 2 |
| Sort...

See http://bugs.mysql.com/bug.php?id=72815 - new upstream bug created for this.

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PXC-1686

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-1505