Question

I have a table, TBL1, with only GUIDs.

I have another table, TBL2, in which the primary key is GUID and it also has some other columns. I want to update one of the columns in TBL2 table based on whether the GUID is in TBL1.

Which of the following queries is faster and/or more reliable to use for that?

MERGE INTO [db].[dbo].[TBL1] AS target
    USING [db].[dbo].[TBL2] as source
        ON target.GUID = source.GUID
    WHEN MATCHED THEN
        UPDATE SET
            StatusColumn = 0;

or

UPDATE [db].[dbo].[TBL1]
    SET StatusColumn=0
    WHERE GUID IN (SELECT GUID FROM [db].[dbo].[TBL2])

or maybe something else?

Answer 1

The answer to this question can only come from the execution plan. From the plan you posted (http://i.imgur.com/6vB2t.png) we can see the following:

• IN is producing a left semi join. This is a little more efficient. There is also an optimizer weakness which causes the optimizer not to generate a semi join from an explicit join even if it could.
• Merge is sorting rows. This is because you might get duplicates from your join! If that was impossible merge would be just as fast.
• I guess the explicit join version is exactly as fast as merge.

Diagnosing this without a plan is just guessing. Look at the plan and/or measure. The measuring provides the answer, but the plan provides the understanding of the answer.