Mysql如何避免全表扫描的方法
如果MySQL需要做一次全表扫描来处理查询时,在 EXPLAIN 的结果中 type 字段的值是 ALL。
在以下几种条件下,MySQL就会做全表扫描:1>数据表是在太小了,做一次全表扫描比做索引键的查找来得快多了。当表的记录总数小于10且记录长度比较短时通常这么做。
2>没有合适用于 ON 或 WHERE 分句的索引字段。
3>让索引字段和常量值比较,MySQL已经计算(基于索引树)到常量覆盖了数据表的很大部分,因此做全表扫描应该会来得更快。
4>通过其他字段使用了一个基数很小(很多记录匹配索引键值)的索引键。这种情况下,MySQL认为使用索引键需要大量查找,还不如全表扫描来得更快。
对于小表来说,全表扫描通常更合适。但是对大表来说,尝试使用以下技术来避免让优化程序错误地选择全表扫描:
1>执行 ANALYZE TABLE tbl_name 更新要扫描的表的索引键分布。
2> 使用 FORCE INDEX 告诉MySQL,做全表扫描的话会比利用给定的索引更浪费资源。
SELECT * FROM t1, t2 FORCE INDEX (index_for_column)
WHERE t1.col_name=t2.col_name;
3>启动 mysqld 时使用参数 --max-seeks-for-key=1000 或者执行 SET max_seeks_for_key=1000 来告诉优化程序,所有的索引都不会导致超过1000次的索引搜索。

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