Introduction

In the world of SQL Server performance troubleshooting, developers and DBAs often chase the usual suspects: bad indexes, poorly written queries, outdated statistics, or parameter sniffing. These are the headline-grabbers that fill blog posts and conference talks. But lurking beneath the surface is another silent killer — one that doesn’t show up in execution plans, that doesn’t leave obvious fingerprints in the code, and that quietly sabotages concurrency: implicit transactions.

Unlike explicit transactions, where the developer deliberately issues a BEGIN TRAN and follows it with a COMMIT or ROLLBACK, implicit transactions begin automatically the moment you run a statement. Unless you know how to end them, they sit open in the background, holding locks far longer than anyone expects. The developer who ran the statement may not even realize they’re running inside a transaction — yet the database engine treats it no differently than a manually opened transaction.

The consequences can be brutal. One “innocent” insert in a session with implicit transactions turned on can block dozens of other sessions, causing queries to hang for minutes, locks to escalate, and even deadlocks to form. Worse, because there’s no obvious BEGIN TRAN in the code, troubleshooting becomes a nightmare: the system looks fine on the surface, but DMV checks reveal open transactions that no one thinks exist.

In this article, we’ll dive into what implicit transactions are, how they get enabled (sometimes without the developer realizing), the real-world problems they cause, and most importantly, how to detect and fix them before they choke your system. We’ll walk through live demos, inspect DMV output, and show practical techniques to prevent accidental misuse. As you’ll see, this one checkbox or session setting can mean the difference between smooth, predictable performance and a system stuck in invisible blockers.

What Are Implicit Transactions?

By default, SQL Server operates in autocommit mode — every statement is treated as a separate transaction that begins, executes, and commits on its own. You issue an INSERT, it commits. You run an UPDATE, it commits.

But when implicit transactions are enabled, SQL Server automatically starts a transaction after the first statement, and you must manually issue COMMIT to finish it. That’s the key difference — it opens the transaction behind the scenes, but does not close it for you.

This can happen in interactive sessions, applications, or through misconfigured clients. The worst part? Developers may not even realize they are inside a transaction — and the longer that uncommitted work lives, the longer locks are held, and the more trouble it causes.

How Implicit Transactions Get Enabled (Accidentally or Not)

There are multiple ways implicit transactions can sneak into your environment:

• • SET IMPLICIT_TRANSACTIONS ON — explicitly used by scripts or tools
  • SSMS settings — the “Implicit Transactions” checkbox under Query Options (see the image below)
  • ODBC/OLEDB behavior — older drivers may default to implicit transactions
  • Third-party applications — some middleware layers wrap everything in a transaction for rollback logic

You can check the session’s transaction behavior using:

DBCC USEROPTIONS;

This will show implicit_transactions = SET like here below,  if you’re in implicit mode.

Demonstrating the Problem: A Simple Blocker

Here’s a quick way to see implicit transactions causing real pain.

Run this code in Session 1:

SET IMPLICIT_TRANSACTIONS ON;
INSERT INTO dbo.TestScores (Score) VALUES (77.5);
-- No COMMIT

Run this in Session 2:

SELECT * FROM dbo.TestScores WITH (TABLOCKX);

This hangs, waiting for session 1 to commit or rollback. Why? Because session 1’s transaction is still open and holds a lock. But there’s no BEGIN TRAN or obvious clue that something’s wrong.

What’s happening? Session 2 is asking for an exclusive table lock. But Session 1 is still holding locks from its uncommitted insert. Because implicit transactions stay open until you explicitly COMMIT or ROLLBACK, Session 2 can’t move forward.

There are a few reasons why this is nasty

• There’s no BEGIN TRAN in Session 1 — so it doesn’t look like a transaction is open.

SSMS doesn’t warn you that you’re holding locks.

Unless you know IMPLICIT_TRANSACTIONS is on, it feels like SQL Server just froze.

The blocking chain only clears if you go back to Session 1 and do either of these things:

COMMIT;   -- make changes permanent
-- or
ROLLBACK; -- undo the changes

Until then, Session 2 (and any other queries touching the same table) will hang behind that invisible transaction.

How It Hurts Performance

Uncommitted transactions hold locks: shared locks, exclusive locks, intent locks. That blocks other readers or writers. If you have dozens or hundreds of sessions doing this unknowingly, your concurrency suffers. Blocking chains form. Tempdb grows (due to version store for RCSI). Log usage increases because nothing can clear until commit. Backups may stall waiting for transactions to close.

Even worse, implicit transactions mess with tools like snapshot isolation or readable secondaries, because the engine doesn’t know when the work will be finalized. And since most monitoring tools don’t track transaction age unless explicitly queried, these transactions silently accumulate in the background.

How to Detect Open Transactions and Session Behavior

Use this DMV to catch sessions with lingering uncommitted work:

SELECT
    s.session_id,
    r.status,
    r.command,
    s.open_transaction_count,
    r.blocking_session_id,
    r.wait_type,
    t.transaction_begin_time
FROM sys.dm_exec_sessions s
JOIN sys.dm_exec_requests r ON s.session_id = r.session_id
JOIN sys.dm_tran_session_transactions st ON s.session_id = st.session_id
JOIN sys.dm_tran_active_transactions t ON st.transaction_id = t.transaction_id
WHERE s.is_user_process = 1;

Look for sessions with open_transaction_count > 0 and long durations.

Also, monitor query text to see who’s running what without closing transactions:

SELECT s.session_id, r.text
FROM sys.dm_exec_requests AS r
CROSS APPLY sys.dm_exec_sql_text(r.sql_handle) AS s
WHERE r.status = 'running';

Fixing the Root Problem

The best fix is education and prevention. Don’t use implicit transactions unless absolutely necessary.

• Make sure SSMS “Implicit Transactions” is OFF in Query Options
• Avoid using SET IMPLICIT_TRANSACTIONS ON in stored procedures
• Use explicit transactions (BEGIN TRAN, COMMIT) only when needed
• Audit client driver settings (especially legacy .NET or Java apps)
• Monitor open transactions in monitoring tools like sp_WhoIsActive or custom scripts

If you suspect implicit mode is being turned on by a driver, use Extended Events or server-side traces to capture login events and session settings.

Final Thoughts

Implicit transactions are rarely the root cause of slowness — but they’re often the quiet enabler of it. Blocking chains, long-lived locks, and weird tempdb behavior may all trace back to this setting. The tricky part is that implicit transactions don’t throw errors, they just don’t commit. And that’s the problem. Once you become aware of how they work and how easily they sneak in, you’ll start spotting them everywhere.

In SQL Server, transparency is power. Implicit transactions hide intent. So next time you see a system slowing down with no obvious reason, check the basics — and maybe check if implicit is quietly choking your workload.