问题
We're just starting to look at moving to SQL 2008 from SQL 2000 and are noting the new CROSS APPLY and INNER APPLY syntax that allows a form of 'joining' against either table-valued parametrized UDFs or correlated subqueries.
Obviously it would be nice to be able to encapsulate logic in a UDF and be able to reuse it in different queries, but I'm sure that functionality comes with a cost.
I've looked around on the Net quite a bit but I can't find any performance metrics that indicate how much of a performance hit you would take when using APPLY-based queries over what you might get if you inlined the same query.
I know the exact impacts will depend heavily on the specific schema and queries, but I was curious if anyone has any experience from tuning real-world systems to share on this.
回答1:
I use APPLY in places. It's useful to force row by row processing instead of a loop, if you have to do it.
Oddly, in one place it ws more efficient (when using profiler to view reads) because the optimiser treated the UDF as a black box and applied the filter as I wanted.
The udf is an inline table valued function that when expanded/unnested in a JOIN was worse because the optmiser looked at the query as a whole and applied the filter ina different place.
Otherwise, I use it sparingly or on code that does not run often... and verify that it does not kill really bad. I accept the hit to gain maintainability.
Off topic: encapsulation only goes so far in databases: at some point you lose the set based advantage.
来源：https://stackoverflow.com/questions/679137/relative-efficiency-of-join-vs-apply-in-microsoft-sql-server-2008