Sql-server – CROSS APPLY on Scalar function

cross-applysql server

I have this:

SELECT
A
,B
,dbo.Func(C)
,dbo.Func(D)
,dbo.Func(E)
,F
FROM abcdef
WHERE
0 = dbo.Func(C) + dbo.Func(D)

I have read that this is not good practice because the function is called million of times and it has a bad impact on performance.

I tried to rewrite it with CROSS APPLY:

SELECT *
 FROM abcdef
  CROSS APPLY dbo.Func(D) as FD

but it returns this error:

Invalid object name 'dbo.Func'

Can I use CROSS APPLY only on TABLE VALUED functions ?

And is CROSS APPLY suitable for scalar functions (converted to table valued function) anyway ? Because on my small performance test CROSS APPLY is a bit slower.

Best Answer

I read that this is not good practice because function is called 'zilion' times and it have bad impact on performance.

While CROSS APPLY can be useful in some cases, I don't expect any difference in performance between calling the function in WHERE or CROSS APPLY in the specific case. If the table has a million rows (and columns C and D possibly a million different values), a million times the function will be called. How can it be otherwise?

I tried to rewrite it with CROSS APPLY.

Here's how:

SELECT
    t.A,
    t.B,
    ca.Fc,
    ca.Fd,
    dbo.Func(t.E) AS Fe
    t.F,
FROM abcdef AS t
  CROSS APPLY 
    ( SELECT
          dbo.Func(t.C) AS Fc, 
          dbo.Func(t.D) AS Fd
    ) AS ca
WHERE 0 = ca.Fc + ca.Fd ;

or:

SELECT
...
FROM abcdef AS t
  CROSS APPLY 
    ( SELECT
          dbo.Func(t.C) AS Fc, 
          dbo.Func(t.D) AS Fd
      FROM (SELECT NULL) AS dummy
      WHERE 0 = dbo.Func(t.C) + dbo.Func(t.d) 
    ) AS ca ;

Again, I don't think this will have any effects on efficiency.

Related Solutions

Sql-server – Cross Apply without a function

SELECT 
  GroupValue = Val, 
  [Count] = COUNT(DISTINCT MyGroup) 
FROM 
(
  SELECT MyGroup, Val = STUFF((SELECT ', ' + RTRIM(MyValue) 
   FROM dbo.MyTable 
   WHERE MyGroup = t.MyGroup 
  FOR XML PATH(''), TYPE).value('.[1]','nvarchar(max)', 1, 2, '')
  FROM dbo.MyTable AS t
) AS x 
GROUP BY Val;

Sql-server – Why would call to scalar function inside a Table Value Function be slower than outside the TVF

Scalar functions are called once-per-row, when called as part of a query.

Consider the following example.

Create a new, blank database for our tests:

USE master;
IF EXISTS (SELECT 1 FROM sys.databases d WHERE d.name = 'mv')
BEGIN
    ALTER DATABASE mv SET SINGLE_USER WITH ROLLBACK IMMEDIATE;
    DROP DATABASE mv;
END
GO
CREATE DATABASE mv;
GO

Create a table, a multi-statement function, and a table-valued-function:

USE mv;
GO
CREATE TABLE dbo.t
(
    t_id int NOT NULL
        CONSTRAINT PK_t
        PRIMARY KEY CLUSTERED
);
GO

CREATE FUNCTION dbo.t_func
(
    @t_id int
)
RETURNS bit
WITH SCHEMABINDING
AS
BEGIN
    DECLARE @r bit;
    IF EXISTS (SELECT 1 FROM dbo.t WHERE t.t_id = @t_id)
        SET @r = 1
    ELSE
        SET @r = 0;
    RETURN @r;
END
GO

CREATE FUNCTION dbo.t_tvf
(
    @min_t_id int
    , @max_t_id int
)
RETURNS TABLE 
WITH SCHEMABINDING
AS
RETURN (
    SELECT t_id = t.t_id
        , e = dbo.t_func(dbo.t.t_id)
    FROM dbo.t
    WHERE t.t_id >= @min_t_id
        AND t.t_id <= @max_t_id
);
GO

Insert some sample data into the table:

INSERT INTO dbo.t (t_id)
SELECT ROW_NUMBER() OVER (ORDER BY c.id, c.colid)
FROM sys.syscolumns c;
GO

Create a table to store function execution stats, and populate it with a start-row showing execution counts for the multi-statement-function, t_func:

CREATE TABLE dbo.function_stats
(
    run_num int NOT NULL
    , object_name sysname NOT NULL
    , execution_count int NULL 
    , CONSTRAINT PK_function_stats
        PRIMARY KEY CLUSTERED (run_num, object_name)
);
GO
INSERT INTO dbo.function_stats (run_num, object_name, execution_count)
SELECT 1
    , o.name
    , COALESCE(fs.execution_count, 0)
FROM sys.objects o 
    LEFT JOIN sys.dm_exec_function_stats fs ON fs.object_id = o.object_id
WHERE o.name = 't_func';
GO

Run a query against the TVF:
```
SELECT t.*
FROM dbo.t_tvf(1, 2) t;
GO
```

Capture the execution stats now:

INSERT INTO dbo.function_stats (run_num, object_name, execution_count)
SELECT 2
    , o.name
    , COALESCE(fs.execution_count, 0)
FROM sys.objects o 
    LEFT JOIN sys.dm_exec_function_stats fs ON fs.object_id = o.object_id
WHERE o.name = 't_func';

The function stats results:

SELECT *
FROM dbo.function_stats fs
ORDER BY fs.run_num
    , fs.object_name;

╔═════════╦═════════════╦═════════════════╗
║ run_num ║ object_name ║ execution_count ║
╠═════════╬═════════════╬═════════════════╣
║       1 ║ t_func      ║               0 ║
║       2 ║ t_func      ║               2 ║
╚═════════╩═════════════╩═════════════════╝

As you can see, the multi-statement-function has execute twice, once per row for the source table accessed by the TVF.

I expect the mutli-statement-function is being called many, many times by the TVF, giving the impression that it is running slowly, whereas in fact it is simply being called many times.

Best Answer

Related Solutions

Sql-server – Cross Apply without a function

Sql-server – Why would call to scalar function inside a Table Value Function be slower than outside the TVF

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