MySQL Notes

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MySQL pagination query

Usage of limit

• The limit clause can be used to force the select statement to return the specified number of records;

• Accepts 1 or 2 integer constants as parameters, the first is the offset of the returned record row , and the second is the maximum number of returned record rows

  -- 检索记录行6~15；
  mysql> SELECT * FROM table LIMIT 5,10;
  -- 从某个偏移量到记录集的最后，可以指定第二个参数为-1；
  mysql> SELECT * FROM table LIMIT 95,-1;
  -- 如果只给定一个参数，表示返回的作答记录行数目;
  mysql> SELECT * FROM table LIMIT 5; 
  -- LIMIT n 等价于 LIMIT 0,n;

  Performance Analysis of MySQL Paging Query Statement

• Make sure to use an index, the following statement creates an index on the ta_id and id columns

  mysql> SELECT * FROM ta WHERE ta_id = 111 ORDER BY id LIMIT 50,10;

• Using subqueries , as the amount of data increases, the offset of the LIMIT statement will be larger and the speed will be significantly slower;

  -- 使用子查询之前，偏移量为10000;
  mysql> SELECT * FROM ta WHERE ta_id = 123 ORDER BY id LIMIT 10000, 10;
  -- 使用子查询之后;
  mysql>
  SELECT * FROM ta WHERE id >=
  (SELECT id FROM ta WHERE ta_id = 111 ORDER BY id LIMIT 10000,1)
  LIMIT 10;

For MySQL tables with large amounts of data, LIMIT paging has serious performance problems:

Optimization method:

• Use subqueries
• Use a similar strategy mode to process paging, and judge that the basic paging method is used within 100 pages, and the sub-query paging method is used for more than 100 pages;
• Order by operation using indexed columns or primary key columns
• Record the primary key returned last time, and use the primary key to filter the next query
• In order to ensure that the index index column is continuous, you can add an auto-incrementing field to each table and add an index