Sql-server – CROSS APPLY versus INNER JOIN

join;sql serversubquery

I was reading a lot about CROSS APPLY versus INNER JOIN recently. I tried writing some queries and they both work in a similar fashion, the execution plan seems to be same as well.

Maybe someone with a deeper knowledge could explain to me what the difference is between these two?

OUTER APPLY (SELECT UserID
                , ActionPerformedDate = MAX(ActionDate)
            FROM dbo.AdminUsage
            WHERE SubscriberID = S.SubscriberID
                AND ActionPerformed = 'Some Action'
                AND Description = 'True'
            GROUP BY UserID) AS AU


LEFT JOIN (SELECT SubscriberID
                , UserID
                , MAX(ActionDate) OVER(PARTITION BY SubscriberID) AS ActionPerformedDate
            FROM dbo.AdminUsage
            WHERE ActionPerformed = 'Some Action'
                AND Description = 'True') AS AU
    ON AU.SubscriberID = S.SubscriberID

Is there a preferred approach in this case?

Best Answer

They're two slightly different animals that can be used for the same purposes, as in your example. CROSS APPLY is your only option for "joining" table value functions and "expanding" xml documents, though.

Some queries, particularly parallel queries, can exhibit vastly improved performance using CROSS APPLY, provided you have the requisite processor threads and indexing strategy.

Microsoft MVP Itzik Ben-Gan elaborates a couple of great examples in this talk

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Sql-server – Outer Apply vs Left Join Performance

Can anyone tell how exactly apply works and how will it effect the performance in very large data

APPLY is a correlated join (called a LATERAL JOIN in some products and newer versions of the SQL Standard). Like any logical construction, it has no direct impact on performance. In principle, we should be able to write a query using any logically equivalent syntax, and the optimizer would transform our input into exactly the same physical execution plan.

Of course, this would require the optimizer to know every possible transformation, and to have the time to consider each one. This process might well take longer than the current age of the universe, so most commercial products do not take this approach. Therefore, query syntax can, and often does, have an impact on final performance, though it is difficult to make general statements about which is better and why.

The specific form of OUTER APPLY ( SELECT TOP ... ) is most likely to result in a correlated nested loops join in current versions of SQL Server, because the optimizer does not contain logic to transform this pattern to an equivalent JOIN. Correlated nested loops join may not perform well if the outer input is large, and the inner input is unindexed, or the pages needed are not already in memory. In addition, specific elements of the optimizer's cost model mean a correlated nested loops join is less likely than a semantically-identical JOIN to produce a parallel execution plan.

I was able to make same query with single left join and row_number()

This may or may not be better in the general case. You will need to performance test both alternatives with representative data. The LEFT JOIN and ROW_NUMBER certainly has potential to be more efficient, but it depends on the precise query plan shape chosen. The primary factors that affect the efficiency of this approach is the availability of an index to cover the columns needed, and to supply the order needed by the PARTITION BY and ORDER BY clauses. A second factor is the size of the table. An efficient and well-indexed APPLY can out-perform a ROW_NUMBER with optimal indexing if the query touches a relatively small portion of the table concerned. Testing is needed.

Sql-server – Outer self join filters vs subqueries

Your first query works as an inner join because the y.TransType = 'used' condition which uses the right table is in the where clause.

Your second query can be rewritten without derived tables by simply moving that condition to the on clause:

select x.*, y.*
from factI as x
left join factI as y on  x.tickedId = y.tickedId
                     and y.TransType = 'used'
where x.TransType = 'sold' ;

Best Answer

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