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  1. MariaDB Server
  2. MDEV-4240

mariadb 5.3.12 using more memory than MySQL 5.1 for an inefficient query

    Details

    • Type: Bug
    • Status: Closed (View Workflow)
    • Priority: Minor
    • Resolution: Fixed
    • Affects Version/s: 5.3.12
    • Fix Version/s: 5.3.13
    • Component/s: None
    • Labels:
    • Environment:
      RedHat EL6 using mariadb-5.3.12-Linux-x86_64.tar.gz

      Description

      Hello and thank you for mariadb,

      I noticed mariadb 5.3.12 using a lot more memory than MySQL 5.1.48 for a very inefficient query (that should never get near production):

      CREATE TABLE `table_a` (
        `field_c` varchar(8) DEFAULT NULL,
        `field_d` varchar(11) DEFAULT NULL,
        UNIQUE KEY `field_c` (`field_c`),
        UNIQUE KEY `field_d` (`field_d`)
      ) ENGINE=InnoDB DEFAULT CHARSET=latin1

      CREATE TABLE `table_b` (
        `field_c` char(9) NOT NULL,
        `field_d` char(12) DEFAULT NULL,
        PRIMARY KEY (`field_c`),
        KEY `field_d` (`field_d`)
      ) ENGINE=MyISAM DEFAULT CHARSET=latin1

      table_a has 4,834,708 rows,
      table_b has 6,522,728 rows, I will upload a tar.gz with mysqldumps of the tables.

      The query:

      SELECT
        table_a.field_d
      FROM
        table_a
        INNER JOIN table_b
          ON table_a.field_c = SUBSTR(table_b.field_c, 1, 8) OR table_a.field_d = SUBSTR(table_b.field_d, 1, 11);

      MySQL 5.1.48 takes a while but does return 4,099,991 rows with:
      VmPeak: 9891240 kB
      VmHWM: 7855352 kB
      Staying under a 10 Gigabyte virtual memory ulimit.

      +----+-------------+---------+------+-----------------+------+---------+------+---------+----------+------------------------------------------------+
      | id | select_type | table   | type | possible_keys   | key  | key_len | ref  | rows    | filtered | Extra                                          |
      +----+-------------+---------+------+-----------------+------+---------+------+---------+----------+------------------------------------------------+
      |  1 | SIMPLE      | table_b | ALL  | NULL            | NULL | NULL    | NULL | 6522728 |   100.00 |                                                |
      |  1 | SIMPLE      | table_a | ALL  | field_c,field_d | NULL | NULL    | NULL | 4817421 |   100.00 | Range checked for each record (index map: 0x3) |
      +----+-------------+---------+------+-----------------+------+---------+------+---------+----------+------------------------------------------------+
      select `test`.`table_a`.`field_d` AS `field_d` from `test`.`table_a` join `test`.`table_b` where ((`test`.`table_a`.`field_c` = substr(`test`.`table_b`.`field_c`,1,8)) or (`test`.`table_a`.`field_d` = substr(`test`.`table_b`.`field_d`,1,11)))

      mariadb 5.3.12 hits a 20 Gigabyte virtual memory ulimit, RSS is around 16G.

      +----+-------------+---------+------+-----------------+------+---------+------+---------+----------+------------------------------------------------+
      | id | select_type | table   | type | possible_keys   | key  | key_len | ref  | rows    | filtered | Extra                                          |
      +----+-------------+---------+------+-----------------+------+---------+------+---------+----------+------------------------------------------------+
      |  1 | SIMPLE      | table_b | ALL  | NULL            | NULL | NULL    | NULL | 6522728 |   100.00 |                                                |
      |  1 | SIMPLE      | table_a | ALL  | field_c,field_d | NULL | NULL    | NULL | 4798671 |   100.00 | Range checked for each record (index map: 0x3) |
      +----+-------------+---------+------+-----------------+------+---------+------+---------+----------+------------------------------------------------+
      select `test`.`table_a`.`field_d` AS `field_d` from `test`.`table_a` join `test`.`table_b` where ((`test`.`table_a`.`field_c` = substr(`test`.`table_b`.`field_c`,1,8)) or (`test`.`table_a`.`field_d` = substr(`test`.`table_b`.`field_d`,1,11)))

      I was using the following when reproducing:

      [mysqld]
      key_buffer = 500M
      innodb_buffer_pool_size = 2000M

      Thank you.

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            • Assignee:
              psergey Sergei Petrunia
              Reporter:
              thatsafunnyname Peter (Stig) Edwards
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              • Created:
                Updated:
                Resolved: