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    <p>You also need <code>HOLDLOCK</code>.</p> <p>If the row does exist then your <code>SELECT</code> statement will take out a <code>U</code> lock on at least that row and retain it until the end of the transaction.</p> <p>If the row doesn't exist there is no row to take and hold a <code>U</code> lock in so you aren't locking anything. <code>HOLDLOCK</code> will lock at least the range where the row would fit in.</p> <p>Without <code>HOLDLOCK</code> two concurrent transactions can both do the <code>SELECT</code> for a non existent row. Retain no conflicting locks and both move onto the <code>INSERT</code>.</p> <p>Regarding the repro in your question it seems the "row doesn't exist" issue is a bit more complex than I first thought. </p> <p>If the row previously did exist but has since been logically deleted but still physically exists on the page as a "ghost" record then the <code>U</code> lock can still be taken out on the ghost explaining the blocking that you are seeing.</p> <p>You can use <code>DBCC PAGE</code> to see ghost records as in this slight amend to your code.</p> <pre><code>SET NOCOUNT ON; DROP TABLE ImportSourceMetadata CREATE TABLE ImportSourceMetadata ( SourceId NVARCHAR(50), LastCheckpoint DATETIME, PRIMARY KEY(SourceId) ) BEGIN TRAN SET TRANSACTION ISOLATION LEVEL READ COMMITTED SELECT LastCheckpoint FROM ImportSourceMetadata WITH (UPDLOCK) WHERE SourceId = 'Source' INSERT INTO ImportSourceMetadata VALUES ('Source', '2013-12-21') DECLARE @DBCCPAGE NVARCHAR(100) SELECT TOP 1 @DBCCPAGE = 'DBCC PAGE(0,' + CAST(file_id AS VARCHAR) + ',' + CAST(page_id AS VARCHAR) + ',3) WITH NO_INFOMSGS' FROM ImportSourceMetadata CROSS APPLY sys.fn_physloccracker(%%physloc%%) ROLLBACK TRAN DBCC TRACEON(3604) EXEC (@DBCCPAGE) DBCC TRACEOFF(3604) </code></pre> <p>The SSMS messages tab shows</p> <pre><code>Slot 0 Offset 0x60 Length 31 Record Type = GHOST_DATA_RECORD Record Attributes = NULL_BITMAP VARIABLE_COLUMNS Record Size = 31 Memory Dump @0x000000001215A060 0000000000000000: 3c000c00 00000000 9ba20000 02000001 †&lt;.......¢...... 0000000000000010: 001f0053 006f0075 00720063 006500††††...S.o.u.r.c.e. Slot 0 Column 1 Offset 0x13 Length 12 Length (physical) 12 </code></pre>
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    1. POWhy does my SQL Server UPSERT code sometimes not block?
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    1. USMartin Smith
    1. USMartin Smith
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    1. POWhy does my SQL Server UPSERT code sometimes not block?
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    1. COYour're right, and I was now able to reproduce `HOLDLOCK` being required by recreating the table in question. It remains a mystery to me, however, why the `SELECT`with `UPDLOCK` _does_ indeed take a `U` lock when I run the test for the second time, even though I delete everything from the table. Where's the difference?
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      1. USFabian Schmied
    2. CO@FabianSchmied - Well if the table doesn't have any rows it can't be taking a `U` lock on a row! What resource is it on and what is your table definition including indexes?
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    3. COThe `U` lock is taken on `resource_type` `KEY`. The table has a clustered index (primary key) on the `SourceId` column. I've added a complete sample to the question above.
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      1. USFabian Schmied
 

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