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.