UPDATE-no-op in SQL MERGE statement

问题

I have a table with some persistent data in it. Now when I query it, I also have a pretty complex CTE which computes the values required for the result and I need to insert missing rows into the persistent table. In the end I want to select the result consisting of all the rows identified by the CTE but with the data from the table if they were already in the table, and I need the information whether a row has been just inserted or not.

Simplified this works like this (the following code runs as a normal query if you like to try it):

-- Set-up of test data, this would be the persisted table 
DECLARE @target TABLE (id int NOT NULL PRIMARY KEY) ;
INSERT INTO @target (id) SELECT v.id FROM (VALUES (1), (2)) v(id);

-- START OF THE CODE IN QUESTION
-- The result table variable (will be several columns in the end)
DECLARE @result TABLE (id int NOT NULL, new bit NOT NULL) ;

WITH Source AS (
    -- Imagine a fairly expensive, recursive CTE here
    SELECT * FROM (VALUES (1), (3)) AS Source (id)
)
MERGE INTO @target AS Target
    USING Source
    ON Target.id = Source.id
    -- Perform a no-op on the match to get the output record
    WHEN MATCHED THEN 
        UPDATE SET Target.id=Target.id
    WHEN NOT MATCHED BY TARGET THEN
        INSERT (id) VALUES (SOURCE.id)
    -- select the data to be returned - will be more columns
    OUTPUT source.id, CASE WHEN $action='INSERT' THEN CONVERT(bit, 1) ELSE CONVERT(bit, 0) END
      INTO @result ;

-- Select the result
SELECT * FROM @result;

I don't like the WHEN MATCHED THEN UPDATE part, I'd rather leave the redundant update away but then I don't get the result row in the OUTPUT clause.

Is this the most efficient way to do this kind of completing and returning data?

Or would there be a more efficient solution without MERGE, for instance by pre-computing the result with a SELECT and then perform an INSERT of the rows which are new=0? I have difficulties interpreting the query plan since it basically boils down to a "Clustered Index Merge" which is pretty vague to me performance-wise compared to the separate SELECT followed by INSERT variant. And I wonder if SQL Server (2008 R2 with CU1) is actually smart enough to see that the UPDATE is a no-op (e.g. no write required).

回答1:

You could declare a dummy variable and set its value in the WHEN MATCHED clause.

 DECLARE @dummy int;
 ...
 MERGE
 ...
 WHEN MATCHED THEN
   UPDATE SET @dummy = 0
 ...

I believe it should be less expensive than the actual table update.

来源：https://stackoverflow.com/questions/5316669/update-no-op-in-sql-merge-statement

标签

tsql

sql-server-2008

sql-update

sql-merge