全部博文（43）
分类： Mysql/postgreSQL
2011-05-14 19:14:11
An index contains values from a specified column or columns in a table.If you index more than one column,the column order is very important,because MySQL can only search efficiently on a leftmost prefix of the index.Creating a index on two columns is not the same as creating two separate single-column indexes.
索引包含的内容来自于表的一列或者几列.
如果索引包含多个列(联合索引),那么定义索引时的列顺序是非常重要的.因为MySQL在搜索时,只有索引的左前缀(leftmost prefix)有效.
在两列上构建两个索引和在这两列上构建联合索引的效果是不一样的.
Indexes are implemented in the storage engine layter,not the server layer.Thus, they are not standardized:indexing works slightly differently in each engine, and not all egines support all types of indexes.