表值函数(TVF)与视图

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最佳答案

A parameterless inline TVF and a non materialized View are very similar. A few functional differences that spring to mind are below.

Views

Accepts Parameters               - No
Expanded out by Optimiser        - Yes
Can be Materialized in advance   - Yes (through indexed views)
Is Updatable                     - Yes
Can contain Multiple Statements  - No
Can have triggers                - Yes
Can use side-effecting operator  - Yes

Inline TVFs

Accepts Parameters               - Yes
Expanded out by Optimiser        - Yes
Can be Materialized in advance   - No
Is Updatable                     - Yes
Can contain Multiple Statements  - No
Can have triggers                - No
Can use side-effecting operator  - No

MultiStatement TVFs

Accepts Parameters               - Yes
Expanded out by Optimiser        - No
Can be Materialized in advance   - No
Is Updatable                     - No
Can contain Multiple Statements  - Yes
Can have triggers                - No
Can use side-effecting operator  - No

At runtime Views and Inline TVFs are both inlined and treated similarly to derived tables or CTEs. They may well not be evaluated in their entirety (or even at all in some cases) or may be evaluated multiple times in others. Multistatement TVFs will always be evaluated and stored in the return table type (basically a table variable)

Occasionally the ability to parameterise inline TVFs directly can lead to a better execution plan than the equivalent parameterised query against a view.

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I have found that joins with MultiStatement TVFs perform much better than Views when a PK is specified on the function's return table.

CREATE FUNCTION [FORMREQS].[fnGetFormsStatus] ()
RETURNS


/* Create a PK using two of the columns */
@Indexed TABLE (
[OrgID] [char](8) NOT NULL,
[PkgID] [int] NOT NULL,
[FormID] varchar(5) NOT NULL,
PRIMARY KEY CLUSTERED(OrgID, PkgID)
)
AS
BEGIN
INSERT @Indexed SELECT OrgID, PkgID, FormID FROM FormsTable


RETURN


END

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I generally have a rule of thumb when it comes to deciding whether to convert my SELECT to a VIEW or a TVF.

Does the view take longer than 2 seconds to finish and does it have more than 10,000 records? If YES, turn it into a TVF. If not, leave it alone.

Of course the rule is purely based on performance.

With a TVF, I can use a CROSS APPLY, for example to treat it as a table, but passing a specific value, such as the primary key.

WHERE ID = xxx, where 'xxx' is the value I pass in the SELECT.

Performance is way faster!

If I had a view of the TVF, I would have to allow the view to bring back over 2 million rows just to return less than 1% of that in my SELECTs.

Something to think about.