疑问
表:sl_sales_bill_head 订单抬头表 数据行:8474
表:sl_sales_bill 订单明细 数据行:8839
字段:SALES_BILL_NO 订单号
情况1
没有任何索引 sql语句
EXPLAIN select * from sl_sales_bill_copy1 lb join sl_sales_bill_head_copy1 lh on lh.SALES_BILL_NO = lb.SALES_BILL_NO
lh为主表 lb为子表
改一下sql语句
EXPLAIN select * from sl_sales_bill_head_copy1 lh join sl_sales_bill_copy1 lb on lh.SALES_BILL_NO = lb.SALES_BILL_NO
疑问:为什么sql语句无论主表是哪个 lh都先执行
情况2
sl_sales_bill_head_copy1 的SALES_BILL_NO为主键索引
ALTER TABLE `sl_sales_bill_head_copy1` ADD PRIMARY KEY (`SALES_BILL_NO`) ;
sql语句1:
EXPLAIN select * from sl_sales_bill_head_copy1 lh join sl_sales_bill_copy1 lb on lh.SALES_BILL_NO = lb.SALES_BILL_NO
sql语句2:
EXPLAIN select * from sl_sales_bill_copy1 lb join sl_sales_bill_head_copy1 lh on lh.SALES_BILL_NO = lb.SALES_BILL_NO
疑问:为什么无论怎么通过sql语句改变主表 始终是lb先执行
情况3
EXPLAIN select * from sl_sales_bill_copy1 lb join sl_sales_bill_head_copy1 lh on lh.SALES_BILL_NO = lb.SALES_BILL_NO where lb.SALES_BILL_NO='HP20190410000099'
EXPLAIN select * from sl_sales_bill_copy1 lb join sl_sales_bill_head_copy1 lh on lh.SALES_BILL_NO = lb.SALES_BILL_NO where lh.SALES_BILL_NO='HP20190410000099'
都会正常走索引 同时也是lh先执行
如果改为lb的其他字段
EXPLAIN select * from sl_sales_bill_copy1 lb join sl_sales_bill_head_copy1 lh on lh.SALES_BILL_NO = lb.SALES_BILL_NO where lb.id='0001c3fd44454a65a4122b259283f979'
无索引情况
ID有索引情况
变成了lb先执行