使用EXPLAIN语句获取有关 MySQL 如何处理查询的信息。要使用它，只需在你的SELECT语句前添加关键字EXPLAIN：Use the EXPLAIN statement to get information about how MySQL processes a query. To use it, just add the keyword EXPLAIN to the front of your SELECT statement:

mysql> EXPLAIN SELECT * FROM t1, t2 WHERE t1.i = t2.i;

EXPLAIN将在第 15.8.2 节，“EXPLAIN 语句”中有更详细的讨论。EXPLAIN is discussed in more detail in Section 15.8.2, “EXPLAIN Statement”.

使用 ANALYZE TABLE tbl_name 来更新扫描表的键分布。参见 第 15.7.3.1 节，“ANALYZE TABLE 语句”。Use ANALYZE TABLE tbl_name to update the key distributions for the scanned table. See Section 15.7.3.1, “ANALYZE TABLE Statement”.

对扫描表使用 FORCE INDEX，告诉 MySQL 与使用给定索引相比，表扫描代价非常高： Use FORCE INDEX for the scanned table to tell MySQL that table scans are very expensive compared to using the given index:

SELECT * FROM t1, t2 FORCE INDEX (index_for_column)
WHERE t1.col_name=t2.col_name;

USE INDEX 和 IGNORE INDEX 也可能有用。参见 第 10.9.4 节，“索引提示”。USE INDEX and IGNORE INDEX may also be useful. See Section 10.9.4, “Index Hints”.

全局和表级 STRAIGHT_JOIN。参见 第 15.2.13 节，“SELECT 语句”。Global and table-level STRAIGHT_JOIN. See Section 15.2.13, “SELECT Statement”.

您可以调整全局或线程特定的系统变量。例如，可以使用 mysqld 启动，并使用 --max-seeks-for-key=1000 选项，或者使用 SET max_seeks_for_key=1000 来告诉优化器假设没有键扫描会导致超过 1000 次键查找。请参见 第 7.1.8 节，“服务器系统变量”。You can tune global or thread-specific system variables. For example, start mysqld with the --max-seeks-for-key=1000 option or use SET max_seeks_for_key=1000 to tell the optimizer to assume that no key scan causes more than 1,000 key seeks. See Section 7.1.8, “Server System Variables”.