To find the rows matching a WHERE clause quickly.快速查找与WHERE子句匹配的行。

To eliminate rows from consideration. 不考虑行。If there is a choice between multiple indexes, MySQL normally uses the index that finds the smallest number of rows (the most selective index).如果在多个索引之间有选择的话，MySQL通常使用查找最少行数的索引（最有选择性的索引）。

If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to look up rows. 如果表有多列索引，则优化器可以使用索引的任何最左侧前缀来查找行。For example, if you have a three-column index on (col1, col2, col3), you have indexed search capabilities on (col1), (col1, col2), and (col1, col2, col3). 例如，如果在(col1, col2, col3)上有一个三列索引，则在(col1)、(col1, col2)和(col1, col2, col3)上有索引搜索功能。For more information, see Section 8.3.6, “Multiple-Column Indexes”.有关更多信息，请参阅第8.3.6节，“多列索引”。

To retrieve rows from other tables when performing joins. 在执行联接时从其他表中检索行。MySQL can use indexes on columns more efficiently if they are declared as the same type and size. 如果将列声明为相同的类型和大小，MySQL可以更有效地使用这些列上的索引。In this context, VARCHAR and CHAR are considered the same if they are declared as the same size. 在此上下文中，如果VARCHAR和CHAR声明为相同的大小，则认为它们是相同的。For example, VARCHAR(10) and CHAR(10) are the same size, but VARCHAR(10) and CHAR(15) are not.例如，VARCHAR(10)和CHAR(10)的大小相同，但VARCHAR(10)和CHAR(15)的大小不同。

For comparisons between nonbinary string columns, both columns should use the same character set. 对于非二进制字符串列之间的比较，两列应使用相同的字符集。For example, comparing a utf8 column with a latin1 column precludes use of an index.例如，比较utf8列和latin1列可以避免使用索引。

Comparison of dissimilar columns (comparing a string column to a temporal or numeric column, for example) may prevent use of indexes if values cannot be compared directly without conversion. 比较不同的列（例如，将字符串列与时间或数字列进行比较）可能会阻止使用索引，如果值不能在没有转换的情况下直接比较。For a given value such as 1 in the numeric column, it might compare equal to any number of values in the string column such as '1', ' 1', '00001', or '01.e1'. 对于给定的值（如数字列中的1），它可能与字符串列中的任意数量的值（如'1'、' 1'、'00001'或'01.e1'）进行比较。This rules out use of any indexes for the string column.这就排除了对字符串列使用任何索引的可能性。

To find the MIN() or MAX() value for a specific indexed column key_col. 查找特定索引列key_col的MIN()或MAX()值。This is optimized by a preprocessor that checks whether you are using WHERE key_part_N = constant on all key parts that occur before key_col in the index. 这是通过一个预处理器来优化的，该预处理器检查是否对索引中key_col之前出现的所有键部分使用WHERE key_part_N = constant。In this case, MySQL does a single key lookup for each MIN() or MAX() expression and replaces it with a constant. 在本例中，MySQL对每个MIN()或MAX()表达式执行单键查找，并用常量替换它。If all expressions are replaced with constants, the query returns at once. 如果所有表达式都替换为常量，则查询将立即返回。For example:例如：

SELECT MIN(key_part2),MAX(key_part2)
  FROM tbl_name WHERE key_part1=10;

To sort or group a table if the sorting or grouping is done on a leftmost prefix of a usable index (for example, ORDER BY key_part1, key_part2). 如果排序或分组是在可用索引的最左边前缀上完成的，则对表进行排序或分组（例如，ORDER BY、key_part1、key_part2）。If all key parts are followed by DESC, the key is read in reverse order. 如果所有关键部件后面都跟有DESC，则按相反顺序读取。(Or, if the index is a descending index, the key is read in forward order.) （或者，如果索引是降序索引，则按前进顺序读取键。）See Section 8.2.1.16, “ORDER BY Optimization”, Section 8.2.1.17, “GROUP BY Optimization”, and Section 8.3.13, “Descending Indexes”.参见第8.2.1.16节，“ORDER BY优化”、第8.2.1.17节，“GROUP BY优化”和第8.3.13节，“降序索引”。

In some cases, a query can be optimized to retrieve values without consulting the data rows. 在某些情况下，可以优化查询以检索值，而无需查询数据行。(An index that provides all the necessary results for a query is called a covering index.) （为查询提供所有必要结果的索引称为覆盖索引。）If a query uses from a table only columns that are included in some index, the selected values can be retrieved from the index tree for greater speed:如果查询仅从表中使用包含在某些索引中的列，则可以从索引树中检索所选值以提高速度：

SELECT key_part3 FROM tbl_name
  WHERE key_part1=1