MySQL系列---sql优化

join时以大表为基表

• 查不动

  explain  select u.id as useId, u.open_id as userOpenId, u.name as userName, u.phone as userPhone, s.id as shopId, s.shop_id as shopNum, s.shop_name as shopName from jml_mp_wechatuser u right join jml_shop s on u.id = shopkeeper_id where u.phone = '6D26B768750A90426AAA4CE04FA4CCBB'
  

  

• 查不动

  EXPLAIN select u.id as useId, u.open_id as userOpenId, u.name as userName, u.phone as userPhone, s.id as shopId, s.shop_id as shopNum, s.shop_name as shopName from jml_shop s LEFT JOIN jml_mp_wechatuser u on u.id = s.shopkeeper_id where u.phone = '6D26B768750A90426AAA4CE04FA4CCBB'
  

  

• 5s

  EXPLAIN select u.id as useId, u.open_id as userOpenId, u.name as userName, u.phone as userPhone, s.id as shopId, s.shop_id as shopNum, s.shop_name as shopName from jml_shop s LEFT JOIN jml_mp_wechatuser u on u.open_id = s.shopkeeper_openid where u.phone = '6D26B768750A90426AAA4CE04FA4CCBB'
  

  

• 3s

  explain  select u.id as useId, u.open_id as userOpenId, u.name as userName, u.phone as userPhone, s.id as shopId, s.shop_id as shopNum, s.shop_name as shopName from jml_mp_wechatuser u left join jml_shop s on u.id = shopkeeper_id where u.phone = '6D26B768750A90426AAA4CE04FA4CCBB'
  

  

• 4s

  explain  select u.id as useId, u.open_id as userOpenId, u.name as userName, u.phone as userPhone, s.id as shopId, s.shop_id as shopNum, s.shop_name as shopName from jml_mp_wechatuser u left join jml_shop s on u.open_id = s.shopkeeper_openid where u.phone = '6D26B768750A90426AAA4CE04FA4CCBB'
  

  

A left join B = B right join A； join的原理是以基准表为底，根据on的条件for循环去另一张表找到等值进而关联。所以显而易见，影响join效率的是等值的长度及for循环次数。以大表为基准可以有效减少for循环次数，选取主键关联可以更快的判断等值。

如何join？ on主键+大表为基准！