问题描述
varchar 列上的索引是否会使查询运行速度变慢?我可以把它变成int.我不需要做 LIKE % 比较.
Does index on a varchar column make the query run slower? I can make that be int. and I don't need to do the LIKE % comparison.
推荐答案
varchar 列上的索引是否会使查询运行速度变慢?
Does index on a varchar column make the query run slower?
不，没有.
如果优化器决定使用索引，查询将运行得更快.该表上的 INSERTs/UPDATEs/DELETEs 会更慢，但不太可能引起注意.
No, it does not.
If the optimizer decides to use of the index, the query will run faster. INSERTs/UPDATEs/DELETEs on that table will be slower, but not likely enough to notice.
我不需要做 LIKE % 比较
I don't need to do the LIKE % comparison
注意使用:
LIKE '%whatever%'
...将不使用索引，但以下将:
...will not use an index, but the following will:
LIKE 'whatever%'
关键是字符串左侧的通配符意味着不能使用列上的索引.
The key is wildcarding the lefthand side of the string means that an index on the column can't be used.
还知道 MySQL 限制了为索引 - 对于 MyISAM(InnoDB 为 767 字节)表，它们最长可达 1000 字节.
Also know that MySQL limits the amount of space set aside for indexes - they can be up to 1000 bytes long for MyISAM (767 bytes for InnoDB) tables.
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