mysql索引和explain執行計劃分析

mysql索引：
MyISAM引擎使用B+Tree作爲索引結構，和DB2的索引一樣，葉節點的data域存放的是數據記錄的地址，
InnoDB的引擎的cluster index葉子節點存放的就是數據本身，如果表沒有primary key或者unique  key，
那麼InnoDB默認會以隱藏字段DB_ROW_ID作爲key創建cluster index，二級索引的葉子節點則存放的是
主鍵的值.
問題：cluster index的主要目的是數據頁的物理和索引鍵的順序一致，這樣在獲取一組範圍的數據的時候
可以讀取連續數據頁減少I/O和數據頁數.InnoDB默認以DB_ROW_ID創建cluster index或者人爲創建自增
字段作爲主鍵，這樣可以連續插入數據頁，是可以避免insert或者update等寫性能影響，可是這樣的主鍵
又不是常用被搜索鍵值，所以這樣的cluster index的好處是什麼呢？使用二級索引的時候，豈不是要進行
二次索引搜索？

瞭解索引幫助分析explain輸出，參考官方文檔：
https://dev.mysql.com/doc/refman/5.7/en/explain-output.html

EXPLAIN Join Types：幾種常見類型由好到差：
 const,system-> ref->range->index->All 

測試：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:一行符合條件而且使用主鍵

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官方解釋和例子：
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;

這裏測試使用非主鍵或非唯一鍵，返回匹配某個單獨值的所有行，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就變成了using index，就是覆蓋索引，即所有值都可以從索引中獲得：
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：使用索引讀取了一定範圍的數據

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語句只訪問索引列

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:全表掃描

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.刪除原來bidx索引，在b,c列創建聯合索引create index bcidx on t(b,c);

排序時候可以使用索引：
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)

把聯合索引b改爲範圍查找，索引b後面的字段失效，排序無法使用索引：
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)

排序順序和索引順序不一樣，排序也無法使用索引：
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)