MySQL的EXPLAIN
MySQL的EXPALIN是优化查询语句必不可少的工具，用户通过它可以获得查询计划的相关信息，查看优化器的选择。
EXPLAIN生成的查询计划中显示优化器计划如何执行查询，包括“SELECT”、“INSERT”、“REPLACE”、“UPDATE”和“DELECE”语句。执行EXPLAIN不会修改执行语句涉及的数据，也不会返回执行语句的结果集。例如：
MySQL localhost:3350 ssl world SQL > EXPLAIN SELECT * FROM world.city where id=100\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: city
partitions: NULL
type: const
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: const
rows: 1
filtered: 100
Extra: NULL
1 row in set, 1 warning (0.0036 sec)
Note (code 1003): /* select#1 */ select '100' AS `ID`,'Paraná' AS `Name`,'ARG' AS `CountryCode`,'Entre Rios' AS `District`,'207041' AS `Population` from `world`.`city` where true
<br>
