mysql索引和explain执行计划分析

mysql索引：
MyISAM引擎使用B+Tree作为索引结构，和DB2的索引一样，叶节点的data域存放的是数据记录的地址，
InnoDB的引擎的cluster index叶子节点存放的就是数据本身，如果表没有primary key或者unique key，
那么InnoDB默认会以隐藏字段DB_ROW_ID作为key创建cluster index，二级索引的叶子节点则存放的是
主键的值.
问题：cluster index的主要目的是数据页的物理和索引键的顺序一致，这样在获取一组范围的数据的时候
可以读取连续数据页减少I/O和数据页数.InnoDB默认以DB_ROW_ID创建cluster index或者人为创建自增
字段作为主键，这样可以连续插入数据页，是可以避免insert或者update等写性能影响，可是这样的主键
又不是常用被搜索键值，所以这样的cluster index的好处是什么呢？使用二级索引的时候，岂不是要进行
二次索引搜索？

了解索引帮助分析explain输出，参考官方文档：
https://dev.mysql.com/doc/refman/5.7/en/explain-output.html

EXPLAIN Join Types：几种常见类型由好到差：
const,system-> ref->range->index->All

测试：create table t(a int primary key,b int,c int);
insert into t2 values(1,11,111),(2,22,222),(5,56,565),(9,32,322),(7,23,233);
create index bidx on t(b);

1.const:一行符合条件而且使用主键

mysql> explain select * from t where a=2\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: const
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 4
          ref: const
         rows: 1
     filtered: 100.00
        Extra: NULL
1 row in set, 1 warning (0.00 sec)

2.ref：

ref官方解释和例子：
All rows with matching index values are read from this table for each combination
 of rows from the previous tables. ref is used if the join uses only a leftmost prefix
 of the key or if the key is not a PRIMARY KEY or UNIQUE index (in other words, if 
the join cannot select a single row based on the key value). If the key that is used 
matches only a few rows, this is a good join type
SELECT * FROM ref_table WHERE key_column=expr;
SELECT * FROM ref_table,other_table
  WHERE ref_table.key_column=other_table.column;
SELECT * FROM ref_table,other_table
  WHERE ref_table.key_column_part1=other_table.column
  AND ref_table.key_column_part2=1;

这里测试使用非主键或非唯一键，返回匹配某个单独值的所有行，也是ref：
mysql> explain select * from t where b=22\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: ref
possible_keys: bidx
          key: bidx
      key_len: 5
          ref: const
         rows: 1
     filtered: 100.00
        Extra: NULL
1 row in set, 1 warning (0.00 sec)

如果把*改成a,b，extra就变成了using index，就是覆盖索引，即所有值都可以从索引中获得：
mysql> explain select a,b from t where b=22\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: ref
possible_keys: bidx
          key: bidx
      key_len: 5
          ref: const
         rows: 1
     filtered: 100.00
        Extra: Using index
1 row in set, 1 warning (0.00 sec)

3.rang：使用索引读取了一定范围的数据

Only rows that are in a given range are retrieved, using an index to select the rows. 
The key column in the output row indicates which index is used. The key_len contains 
the longest key part that was used. The ref column is NULL for this type.
 
mysql> explain select * from t where a>3 \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: range
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 4
          ref: NULL
         rows: 3
     filtered: 100.00
        Extra: Using where
1 row in set, 1 warning (0.00 sec)

4.index：SQL语句只访问索引列

mysql> explain select a,b from t \G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: index
possible_keys: NULL
          key: bidx
      key_len: 5
          ref: NULL
         rows: 5
     filtered: 100.00
        Extra: Using index
1 row in set, 1 warning (0.00 sec)

5.All:全表扫描

sql> explain select * from t where c=222\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 5
     filtered: 20.00
        Extra: Using where
1 row in set, 1 warning (0.00 sec)

6.删除原来bidx索引，在b,c列创建联合索引create index bcidx on t(b,c);

排序时候可以使用索引：
mysql> explain select * from t where b=11 order by c\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: ref
possible_keys: bcidx
          key: bcidx
      key_len: 5
          ref: const
         rows: 3
     filtered: 100.00
        Extra: Using where; Using index
1 row in set, 1 warning (0.00 sec)

把联合索引b改为范围查找，索引b后面的字段失效，排序无法使用索引：
mysql> explain select * from t where b>11 order by c\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: range
possible_keys: bcidx
          key: bcidx
      key_len: 5
          ref: NULL
         rows: 4
     filtered: 100.00
        Extra: Using where; Using index; Using filesort
1 row in set, 1 warning (0.00 sec)

排序顺序和索引顺序不一样，排序也无法使用索引：
mysql> explain select * from t order by b desc,c\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: index
possible_keys: NULL
          key: bcidx
      key_len: 10
          ref: NULL
         rows: 7
     filtered: 100.00
        Extra: Using index; Using filesort
1 row in set, 1 warning (0.00 sec)

mysql索引和explain执行计划分析

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