问题
I have an existing SP running on my production server. I have found some significant performance gain from changing IF EXIST(SELECT 1 FROM ) to IF EXIST(SELECT TOP 1 1 FROM ) and IF NOT EXIST(SELECT 1 FROM ) to IF NOT EXIST(SELECT TOP 1 1 FROM ). The only difference is TOP 1 keyword. Just curious to know whether changing this has any side effect?
回答1:
No, there should be no difference. EXISTS bails out as soon as it has found a single matching row. That's why it's always preferred over e.g. (select COUNT(*) from ...) > 0 - a COUNT would force all rows to be considered.
If you create the following four queries:
select * from sys.objects
select top 1 * from sys.objects
select 1 where exists(select * from sys.objects)
select 1 where exists(select top 1 * from sys.objects)
And turn on execution plans, you'll see that the second query generates an execution plan that includes a TOP operator. The 3rd and 4th queries produce identical plans. The TOP is ignored.
回答2:
When you add TOP 1, it does not continue to other rows. This is what makes the difference. Otherwise it will read whole table from start to end.
To check if a record exist or not, you don't need it as you are obviously going to add a where clause. There may be a tiny difference for that. You can get real performance difference by using indexes.
Using TOP matters only with order by.
来源：https://stackoverflow.com/questions/21451148/is-changing-if-existselect-1-from-to-if-existselect-top-1-from-has-any-sid