When Snapshots Begin

Kendra Little explains when a transaction really begins when you are in the snapshot isolation level:

  • 00.000 – Session A sets its isolation level to snapshot

  • 00.001 – Session A explicitly begins a transaction with BEGIN TRAN

  • 00.002 – Session A starts a WAITFOR command for 15 seconds

  • 10.000 – Before the WAITFOR completes, Session B inserts rows into dbo.Table

  • 15.001 – Session A starts a SELECT from dbo.Table, which returns the rows that Session B inserted

This seems wrong, because many of us commonly say things like, “in Snapshot Isolation level, all statements see data consistent with the beginning of the transaction.”

But in this case, Session B inserted the rows after Session A began its transaction using Snapshot Isolation level. So why did Session A see those rows?

Kendra explains the nuance well, so read the whole thing.

Related Posts

Phantom Reads

Arun Sirpal sees not-quite-there-yet transactions: With Halloween around the corner what better topic to discuss than phantom reads. A phantom read occurs when rows have been inserted after a read operation and becomes visible in a follow-up read operation within the same transaction. I will show you what this looks like with an example. Please […]

Read More

Schema-Only Optimized Tables Can Still Roll Back

Chris Adkin investigates whether schema-only memory-optimized tables are logged and whether they support transactions the way other tables do: The statement “There is zero logging when DURABILITY=SCHEMA_ONLY” is not factually correct, its more like a minimally logged operation. What is surprising is the fact that logged as advertised for the in-memory engine should result in […]

Read More

Categories

June 2017
MTWTFSS
« May Jul »
 1234
567891011
12131415161718
19202122232425
2627282930