Multi-statement Table Valued Function vs Inline Table Valued Function

Question

A few examples to show  just incase   Inline Table Valued  CREATE FUNCTION MyNS GetUnshippedOrders   RETURNS TABLE AS  RETURN SELECT a SaleId  a CustomerID  b Qty     FROM Sales Sales a INNER JOIN Sales SaleDetail b         ON a SaleId   b SaleId         INNER JOIN Production Product c ON b ProductID   c ProductID     WHERE a ShipDate IS NULL GO   Multi Statement Table Valued  CREATE FUNCTION MyNS GetLastShipped  CustomerID INT  RETURNS  CustomerOrder TABLE  SaleOrderID    INT         NOT NULL  CustomerID      INT         NOT NULL  OrderDate       DATETIME    NOT NULL  OrderQty        INT         NOT NULL  AS BEGIN     DECLARE  MaxDate DATETIME      SELECT  MaxDate   MAX OrderDate      FROM Sales SalesOrderHeader     WHERE CustomerID    CustomerID      INSERT  CustomerOrder     SELECT a SalesOrderID  a CustomerID  a OrderDate  b OrderQty     FROM Sales SalesOrderHeader a INNER JOIN Sales SalesOrderHeader b         ON a SalesOrderID   b SalesOrderID         INNER JOIN Production Product c ON b ProductID   c ProductID     WHERE a OrderDate    MaxDate         AND a CustomerID    CustomerID     RETURN END GO   Is there an advantage to using one type  in-line or multi statement  over the other  Is there certain scenarios when one is better than the other or are the differences purely syntactical  I realise the two example queries are doing different things but is there a reason I would write them in that way   Reading about them and the advantages differences haven t really been explained

User · Answer

if you are going to do a query you can join in your Inline Table Valued function like:

SELECT
    a.*,b.*
    FROM AAAA a
        INNER JOIN MyNS.GetUnshippedOrders() b ON a.z=b.z

it will incur little overhead and run fine.

if you try to use your the Multi Statement Table Valued in a similar query, you will have performance issues:

SELECT
    x.a,x.b,x.c,(SELECT OrderQty FROM MyNS.GetLastShipped(x.CustomerID)) AS Qty
    FROM xxxx   x

because you will execute the function 1 time for each row returned, as the result set gets large, it will run slower and slower.

User · Answer

Your examples  I think  answer the question very well  The first function can be done as a single select  and is a good reason to use the inline style  The second could probably be done as a single statement  using a sub-query to get the max date   but some coders may find it easier to read or more natural to do it in multiple statements as you have done  Some functions just plain can t get done in one statement  and so require the multi-statement version   I suggest using the simplest  inline  whenever possible  and using multi-statements when necessary  obviously  or when personal preference readability makes it wirth the extra typing

User · Answer

There is another difference  An inline table-valued function can be inserted into  updated  and deleted from - just like a view  Similar restrictions apply - can t update functions using aggregates  can t update calculated columns  and so on

User · Answer

Another case to use a multi line function would be to circumvent sql server from pushing down the where clause   For example  I have a table with a table names and some table names are formatted like C05 2019 and C12 2018 and and all tables formatted that way have the same schema   I wanted to merge all that data into one table and parse out 05 and 12 to a CompNo column and 2018 2019 into a year column   However  there are other tables like ACA StupidTable which I cannot extract CompNo and CompYr and would get a conversion error if I tried   So  my query was in two part  an inner query that returned only tables formatted like  C         then the outer query did a sub-string and int conversion  ie Cast Substring 2  2  as int  as CompNo   All looks good except that sql server decided to put my Cast function before the results were filtered and so I get a mind scrambling conversion error   A multi statement table function may prevent that from happening  since it is basically a  new  table

User · Answer

Internally  SQL Server treats an inline table valued function much like it would a view and treats a multi-statement table valued function similar to how it would a stored procedure   When an inline table-valued function is used as part of an outer query  the query processor expands the UDF definition and generates an execution plan that accesses the underlying objects  using the indexes on these objects   For a multi-statement table valued function  an execution plan is created for the function itself and stored in the execution plan cache  once the function has been executed the first time    If multi-statement table valued functions are used as part of larger queries then the optimiser does not know what the function returns  and so makes some standard assumptions - in effect it assumes that the function will return a single row  and that the returns of the function will be accessed by using a table scan against a table with a single row   Where multi-statement table valued functions can perform poorly is when they return a large number of rows and are joined against in outer queries   The performance issues are primarily down to the fact that the optimiser will produce a plan assuming that a single row is returned  which will not necessarily be the most appropriate plan   As a general rule of thumb we have found that where possible inline table valued functions should be used in preference to multi-statement ones  when the UDF will be used as part of an outer query  due to these potential performance issues

User · Answer

I have not tested this  but a multi statement function caches the result set   There may be cases where there is too much going on for the optimizer to inline the function   For example suppose you have a function that returns a result from different databases depending on what you pass as a  Company Number    Normally  you could create a view with a union all then filter by company number but I found that sometimes sql server pulls back the entire union and is not smart enough to call the one select   A table function can have logic to choose the source

User · Answer

look at Comparing Inline and Multi-Statement Table-Valued Functions you can find good descriptions and performance benchmarks

User · Answer

Maybe in  a very condensed way   ITVF   inline TVF    more if u are DB person  is kind of parameterized view  take a single SELECT st  MTVF   Multi-statement TVF   Developer  creates and load a table variable

User · Answer

In researching Matt s comment  I have revised my original statement  He is correct  there will be a difference in performance between an inline table valued function  ITVF  and a multi-statement table valued function  MSTVF  even if they both simply execute a SELECT statement  SQL Server will treat an ITVF somewhat like a VIEW in that it will calculate an execution plan using the latest statistics on the tables in question  A MSTVF is equivalent to stuffing the entire contents of your SELECT statement into a table variable and then joining to that  Thus  the compiler cannot use any table statistics on the tables in the MSTVF  So  all things being equal   which they rarely are   the ITVF will perform better than the MSTVF  In my tests  the performance difference in completion time was negligible however from a statistics standpoint  it was noticeable   In your case  the two functions are not functionally equivalent  The MSTV function does an extra query each time it is called and  most importantly  filters on the customer id  In a large query  the optimizer would not be able to take advantage of other types of joins as it would need to call the function for each customerId passed  However  if you re-wrote your MSTV function like so   CREATE FUNCTION MyNS GetLastShipped   RETURNS  CustomerOrder TABLE           SaleOrderID    INT         NOT NULL      CustomerID      INT         NOT NULL      OrderDate       DATETIME    NOT NULL      OrderQty        INT         NOT NULL       AS BEGIN     INSERT  CustomerOrder     SELECT a SalesOrderID  a CustomerID  a OrderDate  b OrderQty     FROM Sales SalesOrderHeader a          INNER JOIN Sales SalesOrderHeader b             ON a SalesOrderID   b SalesOrderID         INNER JOIN Production Product c              ON b ProductID   c ProductID     WHERE a OrderDate                             Select Max SH1 OrderDate                          FROM Sales SalesOrderHeader As SH1                         WHERE SH1 CustomerID   A CustomerId                               RETURN END GO   In a query  the optimizer would be able to call that function once and build a better execution plan but it still would not be better than an equivalent  non-parameterized ITVS or a VIEW   ITVFs should be preferred over a MSTVFs when feasible because the datatypes  nullability and collation from the columns in the table whereas you declare those properties in a multi-statement table valued function and  importantly  you will get better execution plans from the ITVF  In my experience  I have not found many circumstances where an ITVF was a better option than a VIEW but mileage may vary   Thanks to Matt   Addition   Since I saw this come up recently  here is an excellent analysis done by Wayne Sheffield comparing the performance difference between Inline Table Valued functions and Multi-Statement functions   His original blog post   Copy on SQL Server Central

[sql] Multi-statement Table Valued Function vs Inline Table Valued Function

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