Mysql之sql语句优化：explain

explain的列分析

id: 代表select 语句的编号, 如果是连接查询,表之间是平等关系, select 编号都是1,从1开始. 如果某select中有子查询,则编号递增.

mysql> explain select goods_id,goods_name from  goods where goods_id in (select goods_id from  goods where cat_id=4) \G*************************** 1. row ***************************           id: 1  select_type: PRIMARY        table:  goods         type: ALLpossible_keys: NULL          key: NULL      key_len: NULL          ref: NULL         rows: 31        Extra: Using where*************************** 2. row ***************************           id: 2  select_type: DEPENDENT SUBQUERY        table:  goods         type: unique_subquerypossible_keys: PRIMARY,cat_id          key: PRIMARY      key_len: 3          ref: func         rows: 1        Extra: Using where2 rows in set (0.00 sec)

select_type: 查询类型

1. simple (不含子查询)
2. primary (含子查询或派生查询)
3. subquery (非from子查询)
4. derived (from型子查询)
5. union
6. union result

table: 查询针对的表

有可能是
实际的表名 如select * from t1;
表的别名 如 select * from t2 as tmp;
derived 如from型子查询时
null 直接计算得结果,不用走表

possible_key: 可能用到的索引

注意: 系统估计可能用的几个索引,但最终,只能用1个.

key : 最终用的索引.

key_len: 使用的索引的最大长度

type列: 是指查询的方式, 非常重要,是分析”查数据过程”的重要依据

可能的值
- all: 意味着从表的第1行,往后,逐行做全表扫描.,运气不好扫描到最后一行.

例: 把goods_name列上的索引去掉, 并根据goods_name来查询

mysql> explain select goods_name from goods where goods_name='诺基亚N85' \G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: goods         type: ALLpossible_keys: NULL          key: NULL      key_len: NULL          ref: NULL         rows: 31        Extra: Using where1 row in set (0.00 sec)

• index: 比all性能稍好一点，
  

  通俗的说: all 扫描所有的数据行,相当于data_all index 扫描所有的索引节点,相当于index_all

２种情况可能出现：
１：索引覆盖的查询情况下, 能利用上索引,但是又必须全索引扫描.

mysql> explain select goods_id from  goods where goods_id=1 or goods_id+1>20\G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table:  goods         type: indexpossible_keys: PRIMARY          key: PRIMARY      key_len: 3          ref: NULL         rows: 31        Extra: Using where; Using index1 row in set (0.00 sec)mysql> explain select goods_id,click_count from  goods where goods_id=1 or goods_id+1>20 \G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table:  goods         type: ALLpossible_keys: PRIMARY          key: NULL      key_len: NULL          ref: NULL         rows: 31        Extra: Using where1 row in set (0.00 sec)

2: 是利用索引来进行排序,但取出所有的节点select goods_id from goods order by goods_id desc;分析: 没有加where条件, 就得取所有索引节点,同时,又没有回行,只取索引节点.再排序,经过所有索引节点.

mysql> explain select goods_id from  goods order by goods_id asc\G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table:  goods         type: indexpossible_keys: NULL          key: PRIMARY      key_len: 3          ref: NULL         rows: 31        Extra: Using index1 row in set (0.00 sec)

• range: 意思是查询时,能根据索引做范围的扫描

mysql> explain select goods_id,goods_name,shop_price from  goods where goodsid >25 \G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table:  goods         type: rangepossible_keys: PRIMARY          key: PRIMARY      key_len: 3          ref: NULL         rows: 8        Extra: Using where1 row in set (0.00 sec)

• ref 意思是指 通过索引列,可以直接引用到某些数据行

mysql> explain select goods_id,goods_name from  goods where cat_id=4 \G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table:  goods         type: refpossible_keys: cat_id          key: cat_id      key_len: 2          ref: const         rows: 3        Extra:1 row in set (0.00 sec)

在这个例子中,通过cat_id索引 指向N行goods数据,来查得结果.

• eq_ref 是指,通过索引列,直接引用某1行数据
  常见于连接查询中

mysql> explain select goods_id,shop_price from  goods innert join ecs_categoy using(cat_id) where goods_id> 25 \G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: innert         type: rangepossible_keys: PRIMARY,cat_id          key: PRIMARY      key_len: 3          ref: NULL         rows: 8        Extra: Using where*************************** 2. row ***************************           id: 1  select_type: SIMPLE        table: ecs_category         type: eq_refpossible_keys: PRIMARY          key: PRIMARY      key_len: 2          ref: shop.innert.cat_id         rows: 1        Extra: Using index2 rows in set (0.00 sec)

• const, system, null 这3个分别指查询优化到常量级别, 甚至不需要查找时间.

一般按照主键来查询时,易出现const,system
或者直接查询某个表达式,不经过表时, 出现NULL

mysql> explain select goods_id,goods_name,click_count from  goods wher_id=4 \G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table:  goods         type: constpossible_keys: PRIMARY          key: PRIMARY      key_len: 3          ref: const         rows: 1        Extra:1 row in set (0.00 sec)mysql> explain select max(goods_id) from  goods \G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: NULL         type: NULL myisam表的max,min,count在表中优化过,不需要\真正查找,为NULLpossible_keys: NULL          key: NULL      key_len: NULL          ref: NULL         rows: NULL        Extra: Select tables optimized away1 row in set (0.00 sec)

ref列 指连接查询时, 表之间的字段引用关系.

mysql> explain select goods_id,cat_name,goods_name from  goods inner join ec_category using(cat_id) where ecs_category.cat_name='' \G*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table:  goods         type: ALLpossible_keys: cat_id          key: NULL      key_len: NULL          ref: NULL         rows: 31        Extra:*************************** 2. row ***************************           id: 1  select_type: SIMPLE        table: ecs_category         type: eq_refpossible_keys: PRIMARY          key: PRIMARY      key_len: 2          ref: shop. goods.cat_id         rows: 1        Extra: Using where2 rows in set (0.00 sec)

rows : 是指估计要扫描多少行.

extra:

1. index: 是指用到了索引覆盖,效率非常高
2. using where 是指光靠索引定位不了,还得where判断一下
3. using temporary 是指用上了临时表, group by 与order by 不同列时,或group by ,order by 别的表的列.
4. using filesort : 文件排序(文件可能在磁盘,也可能在内存), (?????

select sum(shop_price) from goods group by cat_id(???? 这句话,用到了临时表和文件排序)