Posted By: Anonymous
What is the main purpose of using CROSS APPLY?
I have read (vaguely, through posts on the Internet) that
cross apply can be more efficient when selecting over large data sets if you are partitioning. (Paging comes to mind)
I also know that
CROSS APPLY doesn’t require a UDF as the right-table.
INNER JOIN queries (one-to-many relationships), I could rewrite them to use
CROSS APPLY, but they always give me equivalent execution plans.
Can anyone give me a good example of when
CROSS APPLY makes a difference in those cases where
INNER JOIN will work as well?
Here’s a trivial example, where the execution plans are exactly the same. (Show me one where they differ and where
cross apply is faster/more efficient)
create table Company ( companyId int identity(1,1) , companyName varchar(100) , zipcode varchar(10) , constraint PK_Company primary key (companyId) ) GO create table Person ( personId int identity(1,1) , personName varchar(100) , companyId int , constraint FK_Person_CompanyId foreign key (companyId) references dbo.Company(companyId) , constraint PK_Person primary key (personId) ) GO insert Company select 'ABC Company', '19808' union select 'XYZ Company', '08534' union select '123 Company', '10016' insert Person select 'Alan', 1 union select 'Bobby', 1 union select 'Chris', 1 union select 'Xavier', 2 union select 'Yoshi', 2 union select 'Zambrano', 2 union select 'Player 1', 3 union select 'Player 2', 3 union select 'Player 3', 3 /* using CROSS APPLY */ select * from Person p cross apply ( select * from Company c where p.companyid = c.companyId ) Czip /* the equivalent query using INNER JOIN */ select * from Person p inner join Company c on p.companyid = c.companyId
Can anyone give me a good example of when CROSS APPLY makes a difference in those cases where INNER JOIN will work as well?
See the article in my blog for detailed performance comparison:
CROSS APPLY works better on things that have no simple
This one selects
3 last records from
t2 for each record from
SELECT t1.*, t2o.* FROM t1 CROSS APPLY ( SELECT TOP 3 * FROM t2 WHERE t2.t1_id = t1.id ORDER BY t2.rank DESC ) t2o
It cannot be easily formulated with an
INNER JOIN condition.
You could probably do something like that using
CTE‘s and window function:
WITH t2o AS ( SELECT t2.*, ROW_NUMBER() OVER (PARTITION BY t1_id ORDER BY rank) AS rn FROM t2 ) SELECT t1.*, t2o.* FROM t1 INNER JOIN t2o ON t2o.t1_id = t1.id AND t2o.rn <= 3
, but this is less readable and probably less efficient.
master is a table of about
20,000,000 records with a
PRIMARY KEY on
WITH q AS ( SELECT *, ROW_NUMBER() OVER (ORDER BY id) AS rn FROM master ), t AS ( SELECT 1 AS id UNION ALL SELECT 2 ) SELECT * FROM t JOIN q ON q.rn <= t.id
runs for almost
30 seconds, while this one:
WITH t AS ( SELECT 1 AS id UNION ALL SELECT 2 ) SELECT * FROM t CROSS APPLY ( SELECT TOP (t.id) m.* FROM master m ORDER BY id ) q