Hekaton, read committed isolation, and statement-level inconsistency

It’s stated in SQL Server 2014 Books Online that “The following isolation levels are supported for transactions accessing memory-optimized tables: SNAPSHOT, REPEATABLE READ, and SERIALIZABLE”. But later on, on the same page, it’s stated the “The isolation level READ COMMITTED is supported for memory optimized tables with autocommit transactions”. “Autocommit transaction” is just another way to say that individual SQL statements are always atomic, and so an “autocommit transaction” is a single SQL statement that either completely commits or completely rolls back. There’s no way, for example, for a single SQL UPDATE statement to update 10 rows successfully but fail to update 1 row. Either all 11 rows are updated or none of them are.

I had trouble reconciling these two seemingly conflicting statements about Hekaton tables’ isolation level and set out to find out if Hekaton tables can implement “traditional” SQL Server read committed semantics, or if they *support* read committed, but implement it with snapshot semantics. Read committed means that “the latest committed version of a row is read”.

A well-known behavior of SQL Server tables under read committed is known as “statement-level inconsistency”. You can see this behavior by executing a single SELECT statement in a client program running under a debugger, reading a few rows and then stopping the program, and, in a different session, updating, deleted, or inserting rows that have not yet been read (but remember that SQL Server uses 8000 byte buffers for reading and sending rows to the client side). The final resultset usually will reflect the changes that happen *during* the execution of the statement, which includes the reading of the rows from table to buffer to client. If you haven’t seen this behavior demonstrated, come to a class or place where I’m speaking and ask.

Back to Hekaton tables. There are four use cases, and let’s assume the session’s isolation level is read committed, the default.
1. Running in a compiled stored procedure – the isolation level must be specified in the stored procedure definition. Read committed is not supported.
2. Explicit user transactions (i.e. BEGIN TRAN…etc) – attempting to perform a SELECT statement on a Hekaton table without an isolation level hint or with a query hint of readcommitted produces an error.
3. Implicit user transactions (i.e. SET IMPLICIT_TRANSACTIONS ON) – work the same as explicit user transactions with respect to Hekaton tables.
4. Autocommit transactions – attempting to perform a SELECT on a Hekaton table without an isolation level hint succeeds. You aren’t allowed to use the readcommitted query hint even in an autocommit transaction.

We’re interested in use case 4. And, with that use case, in CTP1, using a SELECT statement without a hint produces SNAPSHOT semantics, rather than READ COMMITTED semantics. There is no statement-level inconsistency for Hekaton tables Also with READ COMMITTED SNAPSHOT isolation, the behavior of the READ COMMITTED isolation level (RCSI) is indistinguishable from SNAPSHOT.

Given those experimental results (and perhaps I need to do more experimentation with queries with more complex plans), I’m prepared to conclude until convinced otherwise that Hekaton tables support READ COMMITTED (and RCSI) with autocommit transactions, but *implement* that level with SNAPSHOT semantics. What’s the difference?

With a multi-version storage engine, each version is read as of a well-defined logical read time. With SNAPSHOT isolation, the logical read time is the beginning of the transaction. READ COMMITTED isolation would read the appropriate version with a logical read time equal to the current time, according to the whitepaper “High-Performance Concurrency Control Mechanisms for Main-Memory Databases“. In autocommit transactions, the logical read time always appears to be the beginning of the (one and only) SQL statement. SNAPSHOT and READ COMMITTED and (RCSI) are indistinguishable.

To wrap this up, most folks would consider the Hekaton behavior to be an improvement, as READ COMMITTED statement-level inconsistancy is not usually considered to be a desirable behavior. But it was worth asking the question… So do Hekaton tables really implement only “SNAPSHOT, REPEATABLE READ, and SERIALIZABLE” isolation?

Cheers, Bob

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