使用explain查询select查询语句执行计划

qddf

1.使用explain查询select查询语句执行计划
mysql> select * from baba where name ='fjdsjf';
+------+--------+
| id   | name   |
+------+--------+
|    1 | fjdsjf |
+------+--------+
查询该sql语句的执行计划
mysql> explain select * from baba where name ='fjdsjf' \G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: baba
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 2
        Extra: Using where
1 row in set (0.00 sec)  //该条sql语句没有走索引

增加索引：
mysql> create index index_baba on baba(name);
Query OK, 2 rows affected (0.09 sec)
Records: 2  Duplicates: 0  Warnings: 0
mysql> desc baba;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(30) | YES  | MUL | NULL    |       |
+-------+-------------+------+-----+---------+-------+
mysql> explain select * from baba where name ='fjdsjf' \G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: baba
         type: ref
possible_keys: index_baba
          key: index_baba
      key_len: 33
          ref: const
         rows: 1    //搜索到一行，就找到所
        Extra: Using where

2. 学会使用执行计划帮助
mysql> help explain
Name: 'EXPLAIN'
Description:
Syntax:
EXPLAIN [explain_type] SELECT select_options

explain_type:
    EXTENDED
  | PARTITIONS

Or:

EXPLAIN tbl_name

The EXPLAIN statement can be used either as a way to obtain information
about how MySQL executes a statement, or as a synonym for DESCRIBE:

o When you precede a SELECT statement with the keyword EXPLAIN, MySQL
  displays information from the optimizer about the query execution
  plan. That is, MySQL explains how it would process the statement,
  including information about how tables are joined and in which order.
  EXPLAIN EXTENDED can be used to obtain additional information.

  For information about using EXPLAIN and EXPLAIN EXTENDED to obtain
  query execution plan information, see
  http://dev.mysql.com/doc/refman/5.1/en/using-explain.html.