Details
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Bug
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Status: Closed (View Workflow)
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Critical
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Resolution: Fixed
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10.1.24, 10.2.9, 10.0(EOL), 10.1(EOL), 10.2(EOL)
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None
Description
MariaDB 10.1.x and 10.2.x do not apply " Select tables optimized away" optimization when MAX(col) value is assigned to a user variable. Consider the following primitive test case:
openxs@ao756:~/dbs/maria10.2$ bin/mysql -uroot test
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Reading table information for completion of table and column names
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You can turn off this feature to get a quicker startup with -A
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Welcome to the MariaDB monitor. Commands end with ; or \g.
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Your MariaDB connection id is 8
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Server version: 10.2.9-MariaDB Source distribution
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Copyright (c) 2000, 2017, Oracle, MariaDB Corporation Ab and others.
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Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
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MariaDB [test]> create table test(a int, key(a));
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Query OK, 0 rows affected (0.32 sec)
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MariaDB [test]> insert into test values (1), (2), (3);
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Query OK, 3 rows affected (0.14 sec)
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Records: 3 Duplicates: 0 Warnings: 0
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MariaDB [test]> explain select max(a) from test;
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+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
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| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
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+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
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1 row in set (0.00 sec)
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MariaDB [test]> explain select @a := max(a) from test;
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+------+-------------+-------+-------+---------------+------+---------+------+------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+-------+-------+---------------+------+---------+------+------+-------------+
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| 1 | SIMPLE | test | index | NULL | a | 5 | NULL | 3 | Using index |
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+------+-------------+-------+-------+---------------+------+---------+------+------+-------------+
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1 row in set (0.00 sec)
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MariaDB [test]> explain select max(a) into @a from test;
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+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
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| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
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+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
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1 row in set (0.00 sec)
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Note that SELECT MAX(col) INTO @a FROM t is NOT affected.
With any large table this leads to huge difference in query execution time, as full index scan is slow.
Upstream MySQL 5.6.x is also not affected:
mysql> create table test(a int, key(a));
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Query OK, 0 rows affected (0.70 sec)
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mysql> insert into test values (1), (2), (3);
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Query OK, 3 rows affected (0.26 sec)
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Records: 3 Duplicates: 0 Warnings: 0
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mysql> explain select max(a) from test;
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+----+-------------+-------+------+---------------+------+---------+------+-----
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-+------------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows
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| Extra |
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+----+-------------+-------+------+---------------+------+---------+------+-----
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-+------------------------------+
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| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL
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| Select tables optimized away |
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+----+-------------+-------+------+---------------+------+---------+------+-----
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-+------------------------------+
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1 row in set (0.12 sec)
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mysql> explain select @id := max(a) from test;
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+----+-------------+-------+------+---------------+------+---------+------+-----
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-+------------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows
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| Extra |
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+----+-------------+-------+------+---------------+------+---------+------+-----
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-+------------------------------+
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| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL
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| Select tables optimized away |
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+----+-------------+-------+------+---------------+------+---------+------+-----
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-+------------------------------+
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1 row in set (0.02 sec)
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mysql> show create table test\G
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*************************** 1. row ***************************
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Table: test
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Create Table: CREATE TABLE `test` (
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`a` int(11) DEFAULT NULL,
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KEY `a` (`a`)
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) ENGINE=InnoDB DEFAULT CHARSET=utf8
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1 row in set (0.03 sec)
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mysql> select version();
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+------------+
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| version() |
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+------------+
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| 5.6.29-log |
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+------------+
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1 row in set (0.04 sec)
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