问题
I'm trying to use a query similar to the following query to find differences between two tables (the same table in a DEV database vs a TEST database). Each table has ~30K rows and ~5 columns.
select field1,field2,field3,field4,field5 from dev.dbo.table1
where field1+field2 in ('string1','string2','string3',...,'string50')
except
select field1,field2,field3,field4,field5 from test.dbo.table1
where field1+field2 in ('string1','string2','string3',...,'string50')
field1 is char(5) and field2 is char(1)
This query essentially never terminates.
When i analyze this query using SET SHOWPLAN_ALL ON, I can see there is a nested loop pretty high in the tree. When I change the above query to
select * from dev.dbo.table1
except
select * from test.dbo.table2
The query runs quickly and there is no nested loop in the execution plan.
Can someone help explain this? I don't understand why there would be a drastic difference.
回答1:
My best guess is that the optimizer is doing a poor job of estimating the cardinality (size) of the two tables. Because it underestimates the size, it is generating a poor query plan.
In SQL Server, you can use join hints on except. So, you can get the query you want with:
select field1,field2,field3,field4,field5 from dev.dbo.table1
where field1+field2 in ('string1','string2','string3',...,'string50')
except
select field1,field2,field3,field4,field5 from test.dbo.table1
where field1+field2 in ('string1','string2','string3',...,'string50')
option (hash join, merge join)
This eliminates the option of the nested loop join, choosing a more favorable method.
回答2:
Your first query is slow because you are concatonating fields, which is essentially a function, in the where clause. This happens pretty much all the time you run functions in a where clause. Here is a simpler example. This will be quick.
where myDateTimeField >= @DateValue
and myDateTimeField < dateadd(day, 1, @DateValue)
This is logically the same, but will be slow
where cast(myDateTimeField as date) = @DateValue
来源：https://stackoverflow.com/questions/16084350/sql-except-performance