是否附加一个“AND `columnname` LIKE '%””损害性能？
简单的问题：将这样的内容添加到查询中是否会损害 mysql 的性能，或者它是否会很快得到优化？
AND `name` LIKE '%'
我这里没有可以测试它的大型数据库。
顺便说一句，我想要这个是因为我希望用户能够将一些参数发送到我的服务器端脚本，然后该脚本获取用户指定的准备好的语句并插入参数。我希望用户在搜索时能够省略参数而不影响性能，而 LIKE 通常相对较慢，对吧？
simple question: Does adding something like this to a query hurt performance in mysql or does it quickly get optimized away?
AND `name` LIKE '%'
I've got no big database here that I could test it on.
By the way, I want this because I want users to be able to send some parameters to my server-side script that then grabs the prepared statement specified by the user and inserts the parameters. I want users to be able to omit parameters when searching without hurting the performance, and LIKE normally is relatively slow, right?
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LIKE 确实可能很昂贵，具体取决于
name字段上的索引。您是否尝试过衡量您的查询？另请参见http://dev.mysql.com/doc/refman/5.0 /en/explain.html
然后MySQL会告诉你它是否优化掉了不必要的LIKE子句。
LIKE's can be expensive indeed, depending on the indexes you have on the
namefield. Have you tried to measure your query?See also http://dev.mysql.com/doc/refman/5.0/en/explain.html
Then MySQL will give you indications whether it has optimised the unnecessary LIKE clause away or not.
是的。 LIKE的非常贵。但与大多数事情一样，这取决于数据库的大小。
Yes. LIKE's are very expensive. But as with most things, it depends on the size of your DB.
LIKE肯定会损害性能。要检查的最重要的事情是您是否拥有正确的索引。正在查询的表上有哪些索引？我怀疑，由于您的
LIKE实际上并未执行任何操作，因此它只会被优化，但您需要进行测试来确认这一点。LIKEcan definately hurt performance. The most important thing to check is that you have the proper indexes. What indexes do you have on the tables being queried?I suspect that since your
LIKEisn't actually doing anything it would just be optimized out but you will need to test to confirm this.我同意前面的答案：
LIKE相当昂贵。如果您的表已正确建立索引，则可以使用LIKE另外查询一个小的结果集。不过，与
OR结合使用时，查询将再次变得极其昂贵。此外，仅使用
LIKE来查询索引的VARCHAR字段最多只能带来最小的性能提升。I agree with the previous answers:
LIKEis pretty expensive. If your table is properly indexed, a small result-set can be additionally queried usingLIKE.In conjunction with
ORthe query is going to be brutally expensive again, though.Also, solely using
LIKEto query an indexedVARCHARfield should present minimal performance gain at best.刚刚使用 MyISAM 表对 MySQL 5 进行了测试。
似乎 MySQL 查询引擎优化了诸如 '0'='0' 和 LIKE '%' 之类的东西。
Just tested with MySQL 5 using MyISAM tables.
Seems that MySQL query engine optimizes things like '0'='0' and LIKE '%' away.