CTE是否会改善性能？

Question

with ini as
(
    select ... 
)
select ini.a 
join ini.b 
join ini.c

How many times does the SQL Server engine calculate the results from the ini table ?

My question which I'm trying to answer (with your help) is if the with statement (CTE) improves performance by aliasing the results.

Answer 1

The CTE ini is simply a macro that expands and this use is syntax/clarity only. MSDN says:

Using a CTE offers the advantages of improved readability and ease in maintenance of complex queries

Nothing about performance.

It is evaluated per mention : so three times here which you can see from an execution plan.

For recursive CTEs it's somewhat different as the CTE builds upon itself but it will still be evaluated once per mention

Answer 2

A CTE (common table expression, the part that is wrapped in the "with") is essentially a 1-time view. If you think of it in terms of a temporary view, perhaps the answer will become more clear. As far as I know, the interpreter will simply do the equivalent of copy/pasting whatever is within the CTE into the main query wherever it finds the reference.

I'm sure there are outside instances where it appears to help, but more often than not, I'd assume that the mere presence of a CTE itself is not going to improve the performance of a query. It'll help with readability and re-usability within that single select statement (ie, you won't have to re-type the same sub-query multiple times), but I don't believe it will magically make things run faster (all things being equal). Of course, if your query is structured differently within the CTE than you would have done w/ sub-queries, then it's quite possible the CTE runs faster at that point, but you're now comparing apples to oranges.

Answer 3

I suppose it would also depend on whther you were using it to replace a derived table or a correlated subquery. Performance would be about the same in the first case and probably significantly better in the second if you joined to the CTE rather than just replaced the suquery code with a reference to the CTE. If you used it to replace a where NOT EXISTS clause with a left join to a CTE (in order to find the records in one table but not the other), I'd expect performance to be worse as Where Exists is usually the fastets way to do that type of task. I guess what I'm saying is that performance will still depend on how you use the CTE not just the fact that you generated one.