MySQL

• 查詢同時連線數上限

  mysql> SHOW VARIABLES where Variable_name like 'max_connect%';
  +--------------------+-------+
  | Variable_name      | Value |
  +--------------------+-------+
  | max_connect_errors | 100   |
  | max_connections    | 151   |
  +--------------------+-------+
  2 rows in set (0.00 sec)
  

• 查詢目前的同時連線數

  mysql> show status where `variable_name` = 'Threads_connected';
  +-------------------+-------+
  | Variable_name     | Value |
  +-------------------+-------+
  | Threads_connected | 151   |
  +-------------------+-------+
  1 row in set (0.00 sec)
  

• 查詢目前連線的 process

  mysql> show processlist;
  +----+-------+-----------+-------+---------+------+-------+------------------+
  | Id | User  | Host      | db    | Command | Time | State | Info             |
  +----+-------+-----------+-------+---------+------+-------+------------------+
  |  1 | root  | localhost | NULL  | Query   |    0 | init  | show processlist |
  |  5 | cacti | localhost | cacti | Sleep   |  292 |       | NULL             |
  |  6 | cacti | localhost | cacti | Sleep   |  311 |       | NULL             |
  |  7 | cacti | localhost | cacti | Sleep   |    7 |       | NULL             |
  |  8 | cacti | localhost | cacti | Sleep   |    7 |       | NULL             |
  | 10 | cacti | localhost | cacti | Sleep   |  306 |       | NULL             |
  | 11 | cacti | localhost | cacti | Sleep   |  306 |       | NULL             |
  | 12 | cacti | localhost | cacti | Sleep   |  306 |       | NULL             |
  | 33 | cacti | localhost | cacti | Sleep   |    0 |       | NULL             |
  | 34 | cacti | localhost | cacti | Sleep   |    1 |       | NULL             |
  | 35 | cacti | localhost | cacti | Sleep   |   37 |       | NULL             |
  | 36 | cacti | localhost | cacti | Sleep   |    4 |       | NULL             |
  | 37 | cacti | localhost | cacti | Sleep   |    7 |       | NULL             |
  | 38 | cacti | localhost | cacti | Sleep   |    7 |       | NULL             |
  | 39 | cacti | localhost | cacti | Sleep   |    7 |       | NULL             |
  | 40 | cacti | localhost | cacti | Sleep   |    7 |       | NULL             |
  | 41 | cacti | localhost | cacti | Sleep   |    7 |       | NULL             |
  +----+-------+-----------+-------+---------+------+-------+------------------+
  17 rows in set (0.00 sec)
  

• 查詢目前的連線等待時間
  • [參考] ​https://stackoverflow.com/questions/23399111/safely-keeping-mysql-connections-alive
  • 預設值是 28800 seconds

    mysql> SHOW VARIABLES where Variable_name like 'wait%';
    +---------------+-------+
    | Variable_name | Value |
    +---------------+-------+
    | wait_timeout  | 28800 |
    +---------------+-------+
    1 row in set (0.00 sec)
    

  • 可以 runtime 修改

    mysql> set global wait_timeout=60;
    mysql> set session wait_timeout=60;
    mysql> SHOW VARIABLES where Variable_name like 'wait%';
    +---------------+-------+
    | Variable_name | Value |
    +---------------+-------+
    | wait_timeout  | 60    |
    +---------------+-------+
    1 row in set (0.00 sec)
    

• 查詢資料庫硬碟用量 - ​參考來源

  SELECT table_name AS `Table`, round(((data_length + index_length) / 1024 / 1024), 2) `Size (MB)` FROM information_schema.TABLES WHERE table_schema = "$your_database";
  

• 查詢資料表硬碟用量 - ​參考來源

  SELECT sum(char_length($your_column))/1024/1024 FROM $your_table
  

• ​RENAME TABLE Syntax
• MySQL Table 換資料庫 - 從 DB_A 要搬到 DB_B

  mysql> RENAME TABLE DB_A.old_table TO DB_B.new_table;
  

• MySQL Table 重新命名

  mysql> RENAME TABLE old_table TO new_table;
  

• MySQL Table 互換名字

  mysql> RENAME TABLE old_table TO tmp_table, new_table TO old_table, tmp_table TO new_table;
  

• 顯示 InnoDB 跟 MyISAM 各自儲存了多少資料量

  mysql> SELECT IFNULL(B.engine,'Total') "Storage Engine",
  CONCAT(LPAD(REPLACE(FORMAT(B.DSize/POWER(1024,pw),3),',',''),17,' '),' ',
  SUBSTR(' KMGTP',pw+1,1),'B') "Data Size", CONCAT(LPAD(REPLACE(
  FORMAT(B.ISize/POWER(1024,pw),3),',',''),17,' '),' ',
  SUBSTR(' KMGTP',pw+1,1),'B') "Index Size", CONCAT(LPAD(REPLACE(
  FORMAT(B.TSize/POWER(1024,pw),3),',',''),17,' '),' ',
  SUBSTR(' KMGTP',pw+1,1),'B') "Table Size"
  FROM (SELECT engine,SUM(data_length) DSize,SUM(index_length) ISize,
  SUM(data_length+index_length) TSize FROM information_schema.tables
  WHERE table_schema NOT IN ('mysql','information_schema','performance_schema')
  AND engine IS NOT NULL GROUP BY engine WITH ROLLUP) B,
  (SELECT 3 pw) A ORDER BY TSize;
  

踩到的雷

• 用 InnoDB 會遇到 ibdata1 一直長大
  • 參考：​ibdata1 grows exponentially when innodb_file_per_table is configured

    /var/lib/mysql$ sudo du -s $(pwd)/* | sort -n | tail -n 2
    33232756    /var/lib/mysql/ibdata1     # 32G
    49176852    /var/lib/mysql/dashboard   # 49G
    

• 用 InnoDB 必須下 OPTIMZE 才能完全釋出已經刪除 record 的硬碟空間，而且執行 OPTIMIZE TABLE 時，需要有足夠的硬碟空間（因為會產生一個新的資料表檔案，然後複製資料進去，再刪掉舊的資料表檔案）
  • 參考：​https://blog.longwin.com.tw/2012/03/mysql-myisam-innodb-optimize-2012/