Delete from table with(snapshot)

Question

I was trying to learn querystore. When going through few queries involved in query store one of the query is below

DELETE TOP (1000000)
FROM [sys].[memory_optimized_history_table_1179151246]
 WITH (SNAPSHOT) 

Full query:

(@rowcount INT OUTPUT) DELETE TOP (1000000) FROM [sys].[memory_optimized_history_table_1179151246] WITH (SNAPSHOT) 
OUTPUT 
DELETED.[ColdRoomTemperatureID], DELETED.[ColdRoomSensorNumber], DELETED.[RecordedWhen], DELETED.[Temperature], DELETED.[ValidFrom], DELETED.[ValidTo]  
 INTO [Warehouse].[ColdRoomTemperatures_Archive]  
 WHERE [ValidTo] < GetHkOldestTxTemporalTs() 

I searched a lot to see what the WITH (SNAPSHOT) option means, but I was not able to find anything.

Can you please help me in understanding what does WITH (SNAPSHOT) mean?

Let me know ,if you need any further info.

Answer 1

This is a table hint that can be used in various statements (SELECT, INSERT, DELETE, UPDATE, MERGE). See the related page at msdn: Table hints and the linked Introduction to Memory-Optimized tables.

For further information about isolation levels, see SET TRANSACTION ISOLATION LEVEL (Transact-SQL).

Syntax

WITH  ( <table_hint> [ [, ]...n ] )  
  
<table_hint> ::=   
[ NOEXPAND ] {   
    INDEX  ( index_value [ ,...n ] )   
  | INDEX =  ( index_value )      
  | FORCESEEK [( index_value ( index_column_name  [ ,... ] ) ) ]  
  | FORCESCAN  
  ...
  | SNAPSHOT
  ...

...

SNAPSHOT

Applies to: SQL Server 2014 through SQL Server 2016.

The memory-optimized table is accessed under SNAPSHOT isolation.
SNAPSHOT can only be used with memory-optimized tables (not with disk-based tables).
For more information, see Introduction to Memory-Optimized Tables.