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  • MySql优化子查询

    • 用子查询语句来影响子查询中产生结果rows的数量和顺序. For example:

    • SELECT * FROM t1 WHERE t1.column1 IN
        (SELECT column1 FROM t2 ORDER BY column1);
      SELECT * FROM t1 WHERE t1.column1 IN
        (SELECT DISTINCT column1 FROM t2);
      SELECT * FROM t1 WHERE EXISTS
        (SELECT * FROM t2 LIMIT 1);//limit关键字不在含有in关键字的子查询中(用exists代替)
    • 代替和子查询做join操作. For example:

      SELECT DISTINCT column1 FROM t1 WHERE t1.column1 IN (
        SELECT column1 FROM t2);

      代替:

      SELECT DISTINCT t1.column1 FROM t1, t2
        WHERE t1.column1 = t2.column1;
    • 一些子查询会被改写成join连接为了兼容不支持子查询的老版本.然而,在一些情况下改写子查询为join操作会提高性能 ;

    • 去掉在子查询中出现的外部语句. For example:

      SELECT * FROM t1
        WHERE s1 IN (SELECT s1 FROM t1 UNION ALL SELECT s1 FROM t2);

      代替:

      SELECT * FROM t1
        WHERE s1 IN (SELECT s1 FROM t1) OR s1 IN (SELECT s1 FROM t2);

      For another example:

      SELECT (SELECT column1 + 5 FROM t1) FROM t2;

      代替:

      SELECT (SELECT column1 FROM t1) + 5 FROM t2;
    • 用行子查询代替一个相关子查询. For example:

      SELECT * FROM t1
        WHERE (column1,column2) IN (SELECT column1,column2 FROM t2);

      代替:

      SELECT * FROM t1
        WHERE EXISTS (SELECT * FROM t2 WHERE t2.column1=t1.column1
                      AND t2.column2=t1.column2);
    • 用 NOT (a = ANY (...)) 代替 a <> ALL (...).

    • 用 x = ANY (table containing (1,2))代替 x=1 OR x=2.

    • 用 = ANY 代替 EXISTS.

    • 因为不相关的子查询通常返回一行结果, IN 通常慢于 =. For example:

      SELECT * FROM t1
        WHERE t1.col_name = (SELECT a FROM t2 WHERE b = some_const);

      代替:

      SELECT * FROM t1
        WHERE t1.col_name IN (SELECT a FROM t2 WHERE b = some_const);
    • MySQL 执行不相关的子查询一次. 用 explain确保一个子查询是真正的不相关的.

    • MySQL改写 INALLANY, and SOME 子查询尝试提高select的列在子查询中加索引的可能性 .

    • MySQL 代替用带Index查找功能,explain语句描述为一种特别的join(unqie subquery 或者index subquery)子查询(如下面形式):

      ... IN (SELECT indexed_column FROM single_table ...)
      
    • MySQL 增前了表达式(以下形式调用(min() or max()), 除非null值或者空集合:

      value {ALL|ANY|SOME} {> | < | >= | <=} (uncorrelated subquery)
      

      For example,:

      WHERE 5 > ALL (SELECT x FROM t)
    • 可能被优化成:WHERE 5 > (SELECT MAX(x) FROM t)

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  • 原文地址:https://www.cnblogs.com/onlysun/p/4512193.html
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