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HomeDatabaseMysql TutorialMYSQL WHERE语句优化

where优化主要是在SELECT中,因为他们最主要是在那里使用,但是同样的优化也可被用于DELETE和UPDATE语句。

但请注意,下面的优化并不是完全的。MYSQL实施了许多优化,但我没时间全部测试.
MySQL的一些优化列在下面:
删除不必要的括号:
((a AND b) AND c OR (((a AND b) AND (c AND d))))
-> (a AND b AND c) OR (a AND b AND c AND d)
常数调入:
(a-> b>5 AND b=c AND a=5
删除常数条件:
(B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6)
-> B=5 OR B=6
索引使用的常数表达式仅计算一次。
在一个单个表上的没有一个WHERE的COUNT(*)直接从表中检索信息。当仅使用一个表时,对任何NOT NULL表达式也这样做。
无效常数表达式的早期检测。MySQL快速检测某些SELECT语句是不可能的并且不返回行。
如果你不使用GROUP BY或分组函数(COUNT()、MIN()……),HAVING与WHERE合并。
为每个子联结(sub join),构造一个更简单的WHERE以得到一个更快的WHERE计算并且也尽快跳过记录。
所有常数的表在查询中的在其他任何表之前被读出。
一个常数的表是:
一个空表或一个有1行的表。
与在一个UNIQUE索引、或一个PRIMARY KEY的WHERE子句一起使用的表,这里所有的索引部分使用一个常数表达式并且索引部分被定义为NOT NULL。
所有下列的表用作常数表:
mysql> SELECT * FROM t WHERE primary_key=1;
mysql> SELECT * FROM t1,t2 WHERE t1.primary_key=1 AND t2.primary_key=t1.id;
对联结表的最好联结组合是通过尝试所有可能性来找到:(。如果所有在ORDER BY和GROUP BY的列来自同一个表,那么当联结时,该表首先被选中。
如果你使用SQL_SMALL_RESULT,MySQL将使用一个在内存中的表。
如果有一个ORDER BY子句和一个不同的GROUP BY子句,或如果ORDER BY或GROUP BY包含不是来自联结队列中的第一个表的其他表的列,创建一个临时表。
因为DISTINCT被变换到在所有的列上的一个GROUP BY,DISTINCT与ORDER BY结合也将在许多情况下需要一张临时表。
每个表的索引被查询并且使用跨越少于30% 的行的索引。如果这样的索引没能找到,将使用一个快速的表扫描。
在一些情况下,MySQL能从索引中读出行,甚至不用查询数据文件。如果索引使用的所有列是数字的,那么只有索引树被用来解答查询。
在每个记录被输出前,那些不匹配HAVING子句的行将被跳过。
下面是一些快速的查询例子:
mysql> SELECT COUNT(*) FROM tbl_name;
mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;
mysql> SELECT MAX(key_part2) FROM tbl_name
WHERE key_part_1=constant;
mysql> SELECT ... FROM tbl_name
ORDER BY key_part1,key_part2,... LIMIT 10;
mysql> SELECT ... FROM tbl_name
ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;
下列查询仅使用索引树就可解决(假设索引列是数字的):
mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;
mysql> SELECT COUNT(*) FROM tbl_name
WHERE key_part1=val1 AND key_part2=val2;
mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;
下列查询使用索引以排序顺序检索,不用一次另外的排序:
mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,...
mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,...
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