Blocking Merge Statement

Kendra Little walks through the MERGE command and potential blocking issues with it:

The holdlock hint is a way to get serializable isolation level in SQL Server for a specific table, without having to change the isolation level for your entire session. Serializable is the highest isolation level in SQL Server using pessimistic locking.

When you “HOLDLOCK”, you tell SQL Server to protect any rows you read with a range lock– just in case someone comes along and tries to change one or sneak  one in.

That means that even when you’re just reading ParentTable and not inserting a row, you’re taking out a key range lock. You’re willing to fight other users over those rows to protect your statement.

Kendra’s final advice is to avoid the MERGE command in most cases; read on to find out why.

Related Posts

What Update Locks Do

Guy Glantser explains the process around updating data in SQL Server, particularly the benefit of having update locks: In order to update a row, SQL Server first needs to find that row, and only then it can perform the update. So every UPDATE operation is actually split into two phases – first read, and then […]

Read More

ROWGROUP_FLUSH Deadlocks Inserting Into Clustered Columnstore Indexes

Joe Obbish ran into a strange deadlock when performing concurrent insertions into a clustered columnstore index: We’ve only observed this deadlock with multiple concurrent sessions insert to the delta store for the same target CCI due to server memory pressure or very low cardinality estimates (less than 251 rows). The correct mitigation depends on why […]

Read More

Categories

May 2016
MTWTFSS
« Apr Jun »
 1
2345678
9101112131415
16171819202122
23242526272829
3031