Category Archives: Transaction processing

All about In-Memory isolation levels, Part 2

In the Part 1, we covered the basics of transaction initiation modes and isolation levels. Today we’ll continue with more details about isolation levels and initiation modes for memory-optimized tables, and finally we’ll see how to reference both types of tables in a query.

But first, let’s summarize supported isolation levels.

List 1:


Last time, we had this for “List 2”:


But that’s not the whole truth – the complete picture of isolation levels and initiation modes for memory-optimized tables is summarized in the following table:


In Part 1, we said that READ COMMITED is supported for memory-optimized tables, but we didn’t explain how. Here we can see that it’s supported, but only for single statement, “autocommit” transactions. Autocommit transactions are not possible within a native module, so you’re limited to interpreted TSQL (un-compiled), as indicated in the table above.

Let’s work through an example.

If the transaction isolation level is set to READ COMMITED SNAPSHOT – which, as detailed in the last post, can only be set with an ALTER DATBASE command – then you can execute the following:

That’s a single statement that will be autocommitted.

But you cannot execute the following:

Why will it fail?

It will fail because the initiation mode of this transaction is not autocommit, which is required for READ COMMITED SNAPSHOT when referencing memory-optimized tables (the initiation mode is explicit, because we explicitly defined a transaction).  So to be totally clear, for queries that only reference memory-optimized tables, we can use the READ COMMITTED or READ COMMITTED SNAPSHOT isolation levels, but the transaction initiation mode must be autocommit. Keep this in mind, because in a moment, you’ll be questioning that statement….

Now let’s put it all together and understand the rules for referencing on-disk and memory-optimized tables in the same query.


A query that references both on-disk and memory-optimized tables is known as a “cross-container” transaction.

The following table lists the rules:


If the on-disk or database isolation level is READ UNCOMMITTED, READ COMMITTED, or READ COMMITTED SNAPSHOT, then you can reference memory-optimized tables using SNAPSHOT, REPEATABLE READ, or SERIALIZABLE isolation levels. An example would be:

But wait – a moment ago we proved that when we use the READ COMMITTED isolation level, and we query a memory-optimized table, the transaction initiation mode must be autocommit. The code we just displayed uses an explicit transaction to do the same thing, so we’ve got some explaining to do….

The answer is that for queries that only reference memory-optimized tables, we must use autocommit. But the rules are different for cross-container transactions, and in that case, we can use explicit transactions.


What if we converted some tables to be memory-optimized, and they were referenced everywhere in our TSQL code?

Would we have to change all of our code to use WITH (SNAPSHOT)?

Fear not, there is a solution, and it’s a database setting known as MEMORY_OPTIMIZED_ELEVATE_TO_SNAPSHOT. If this option is enabled, then you don’t have to change your code to use WITH (SNAPSHOT) for interop statements. The engine will automagically elevate the isolation level to SNAPSHOT for all memory-optimized tables that use interop/cross-container. More information on MEMORY_OPTIMIZED_ELEVATE_TO_SNAPSHOT is available at this link.

Just to recap what we covered last time about the different forms of snapshot isolation:

  • READ COMMITTED SNAPSHOT isolation is “statement-level consistency”
  • SNAPSHOT isolation is “transaction-level consistency”

A cross-container transaction doesn’t support snapshot isolation, because it’s actually two sub-transactions, each with its own internal transaction ID and start time. As a result, it’s impossible to synchronize transaction-level consistency between on-disk and memory-optimized tables.

Wrapping up

In the first post on transaction processing, we covered isolation levels for both on-disk and memory-optimized tables, but we left out some of the details for memory-optimized tables. This post has filled in those details, and also introduced the rules for cross-container transactions.

All about In-Memory isolation levels, Part 1


Transaction initiation modes

If you want to understand the details of transaction isolation requirements for memory-optimized tables, you must first understand transaction initiation modes. That’s because the initiation mode affects what type of isolation levels are possible when referencing memory-optimized tables.

There are four different modes that describe the way in which a transaction is initiated:

Atomic Block – An atomic block is a unit of work that occurs within a natively compiled module (procedure, function, or trigger). Native modules can only reference memory-optimized tables.

Explicit – We’re all familiar with this mode, because it requires defining an explicit beginning for the transaction, and then either a commit or rollback.

Implicit – We’ll cover this mode for the sake of completeness, but I’ve not seen an implicit transaction in all my years of SQL Server. Implicit transactions require you to SET IMPLICIT_TRANSACTIONS ON, which then  – for specific types of TSQL statements – has the effect of beginning a transaction for you. It’s only benefit is that it spares you from having to write your own BEGIN TRAN statement (woo hoo).

Documentation for implicit transactions can be found here.

Autocommit – If you execute TSQL statements outside of an explicit or implicit transaction, and outside of an atomic block, then for each individual statement, the SQL Server engine starts a transaction. That transaction is automatically committed or rolled back.

An example of an autocommit transaction would be:

We did not create an explicit transaction with BEGIN TRAN, and we didn’t SET IMPLICIT_TRANSACTIONS ON, which would have allowed the engine to implicitly start a transaction. Therefore, this TSQL statement will be automatically committed or rolled back by the engine.

Isolation levels

Now that we have a basic understanding of transaction initiation modes, let’s move on to isolation levels. Isolation levels are what determine whether certain “concurrency side effects” are allowed, such as dirty reads (uncommitted data), or phantom reads. Please refer to the SQL Server documentation on isolation levels at this link or this link for specific details.

First, let’s display which types of isolation levels are available for each type of table.

List 1:



One thing I want to clear up right way, is how freely the word “snapshot” is used in the SQL Server documentation, the technology world in general, and how confusing this label is in the context of transaction isolation levels.

Some editions of SQL Server have the ability to create database snapshots, which use NTFS sparse files to make a “copy on write”, read-only version of a database. This type of snapshot has absolutely nothing to do with isolation levels.

The word “snapshot” is also used to describe saving the state of a virtual machine, i.e. Hyper-V, VMware, etc.

And there are also SAN snapshots, which create an image of your storage at a fixed point in time. Again, none of these types of snapshots have anything to do with isolation levels in SQL Server.

There are two variations of snapshot isolation in SQL Server:

  • statement-level consistency – Within the context of a transaction, each statement sees consistent data as of the moment the statement executed. Other transactions can modify data while your transaction is executing, potentially affecting results.
  • transaction-level consistency – All data that is referenced within the context of a transaction is guaranteed to be consistent as of the transaction start time. While your transaction is executing, modifications by other transactions cannot be seen by any statement within your transaction. When you attempt to COMMIT there can be conflicts, but we won’t cover that in this post.

Statement-level consistency is known as “read committed snapshot isolation”, while transaction-level consistency is known as “snapshot isolation”. Both can be enabled at the database level, while only transaction-level consistency can be set with the SET TRANSACTION ISOLATION command.


(wrapping your brain around variations of snapshot isolation will help you understand some of the nuances in the next post)

List 2*:

(*READ COMMITTED isolation is supported for memory-optimized tables, and we’ll cover that in the next post, but for now let’s concentrate on the isolations listed here)


If you are only querying on-disk tables, you can use any of the isolations levels from List 1. And if you are only querying memory-optimized tables, you can use any of the isolation levels from List 2.

But what if you want to reference both on-disk and memory-optimized tables in the same query? Of course, the answer is “it depends”, with transaction initiation modes and isolation levels being the components of that dependency.

As mentioned earlier, you can’t use native compilation to reference both on-disk and memory-optimized tables – instead you must use interpreted TSQL, otherwise known as “interop”. In the next post we’ll discuss the requirements for using interop to reference both on-disk and memory-optimized tables within a single query.