譬如 where price_apple>5 and price_orange>5 price_apple>5 这个条件执行比后面的快 原理是什么?
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billlee 2017-06-27 01:14:57 +08:00
你这是从哪里看来的歪理邪说?
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mkdong 2017-06-27 01:23:14 +08:00 via iPhone
可能…加了索引?
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andrewpsy 2017-06-27 01:53:19 +08:00
SQL 水平太差,没看懂 where 句 :<
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yangqi 2017-06-27 03:01:19 +08:00
先把问题表达清楚了。。。
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shiji 2017-06-27 04:39:47 +08:00
用 explain 语句解释一下,你就会发现。。。MySQL 并不管你 where 里面的顺序
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stabc 2017-06-27 05:37:25 +08:00
问为什么之前先搞清楚是什么。
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msg7086 2017-06-27 05:48:44 +08:00
为何你说最左边的执行最快?根据是什么?
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imherer 2017-06-27 11:15:45 +08:00
哪些属于左边?🤣
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guyeuro OP |
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msg7086 2017-06-28 02:40:33 +08:00
节选自标准文档。
ANSI SQL Draft 2003 5WD-01-Framework-2003-09.pdf 6.3.3.3 Rule evaluation order ... Where the precedence is not determined by the Formats or by parentheses, effective evaluation of expressions is generally performed from left to right. However, it is implementation-dependent whether expressions are actually evaluated left to right, particularly when operands or operators might cause conditions to be raised or if the results of the expressions can be determined without completely evaluating all parts of the expression. 说是通常可以选择从左到右进行计算,但是具体是否要从左到右计算,是由数据库软件自行决定的。如果数据库认为优先计算其他表达式可以提高计算速度,那么他可以自行选择求值顺序。 |