两个 mysql SQL优化技艺
两个 mysql SQL优化技巧 1, limit offset 优化 ? ? ? ?mysql SELECT * FROM `bigtable` order by value limit 50,5; ? ? ? ?优化后: ? ? ? ?mysql EXPLAIN SELECT * FROM `bigtable` ? ? ? ? ? ? ? ? INNER JOIN( ? ? ? ? ? ??select id from `bigtable` or
两个 mysql SQL优化技巧1, limit offset 优化
? ? ? ?mysql >SELECT * FROM `bigtable` order by value limit 50,5;
? ? ? ?优化后:
? ? ? ?mysql >EXPLAIN SELECT * FROM `bigtable`
? ? ? ? ? ? ? ? INNER JOIN(
? ? ? ? ? ??select id from `bigtable` order by value limit 50,5
? ? ? ? ? ? ? ? ?) as lim USING(id);
? ? ? ?说明:通过联结先取得符合条件的主键ID,然后再在原表上查找剩余的行,这样减少了读取无用行中的数据。
? ? ? ?如果事先将limit 分页计算出索引列的准确位置,那么可以使用索引范围扫描,写成这样:
? ? ? mysql >SELECT * FROM `bigtable` where position between 5 and 10 order by position;
? ? ? position 是bigtable 表的一列。
2, 始终使用union all 代替union ,除非需要服务器消除重复的行。

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