mysql语句写法性能比较

update tbl_order_sub ts set ts.pay_status = 9
WHERE
ts.id = (
	select tse.order_sub_id from tbl_order_sub_expand tse
	where ts.out_order_id = '10030311646136'
	and ts.pay_status = 30
	and tse.pre_sale_order = 1
	and ts.id = tse.order_sub_id
)

索引:   ts.id , ts.out_order_id

受影响的行: 1
时间: 81.230ms

把ts.id = ()修改成ts.id in ()执行结果时间没有改变

update tbl_order_sub ts set ts.pay_status = 9
WHERE
ts.id in (
	select ts1.id from tbl_order_sub ts1,tbl_order_sub_expand tse
	where ts1.out_order_id = '10030311646136'
	and ts1.id = tse.order_sub_id
	and ts1.pay_status = 30
	and tse.pre_sale_order = 1
)

[Err] 1093 - You can't specify target table 'ts' for update in FROM clause

update tbl_order_sub ts set ts.pay_status = 9
WHERE
ts.out_order_id = '10030311646136'
and 
ts.id = (
	select ts.id from tbl_order_sub_expand tse
	where ts.out_order_id = '10030311646136'
	and ts.id = tse.order_sub_id
	and ts.pay_status = 30
	and tse.pre_sale_order = 1
	LIMIT 1
)

受影响的行: 1
时间: 0.034ms

where条件中每一次where查询，都会先把子查询执行一遍，因此若是能在子查询外进行条件割舍就割舍。

-----------------------------------------------------------------------------------

update tbl_order_sub_expand ts 
		set ts.balance_due_date = now()
		where ts.order_sub_id = 
		( select tos.id from tbl_order_sub tos where tos.out_order_id = '10030311646136'
			and tos.id = ts.order_sub_id
			limit 1
		)

受影响的行: 1
时间: 94.329ms
执行时间为94m,要放在程序里面，那得吓死人了。

这种状况还不如先把order_sub_id的值取出来再进行update操做。在DB端若是是长链接，两次得到connection也不会浪费太多的时间。

-----------------------------------------------------------------------------------

update tbl_order_main om 
set om.prod_total_amt = 100,
om.online_pay_amount = 100
where om.id = (
	select os.order_main_id from tbl_order_sub os
	where os.order_main_id = om.id
	and os.id = '8b76d19ace50484ca6d14790b5c6f5ba'
)

耗时80s

update tbl_order_main om ,tbl_order_sub os
set om.prod_total_amt = 100,
om.online_pay_amount = 100
where os.order_main_id = om.id
	and os.id = '8b76d19ace50484ca6d14790b5c6f5ba'

可能会要给两个表都进行加锁操做

update tbl_order_main om 
set om.prod_total_amt = 100,
om.online_pay_amount = 100
where om.id = (
	select os.order_main_id from tbl_order_sub os
	where os.id = '8b76d19ace50484ca6d14790b5c6f5ba'
	limit 1
)

受影响的行: 0
时间: 0.001ms

减小了一个条件：

where os.order_main_id = om.id

而正是这个条件让sql解释器无法跳过优化，须要每次都执行完整的sql

//exist效率怎么样

select * from tbl_order_main om
where EXISTS(
	select 1 from tbl_order_sub os
	where os.order_main_id = om.id
	and os.original_order_no is NULL
	and os.payment != 2
)
LIMIT 10
;

受影响的行: 0
时间: 0.059ms

select * from tbl_order_main om,tbl_order_sub os
where os.order_main_id = om.id
and os.original_order_no is NULL
and os.payment != 2
LIMIT 10

受影响的行: 0 时间: 0.124ms