READ UNCOMMITTED isolation level in SQL server

READ UNCOMMITTED is the lowest isolation level in SQL Server. It allows a transaction to read data modified by other transactions even if those changes are not yet committed. SQL Server does not acquire Shared Locks for reads and ignores Exclusive Locks held by writers. This eliminates reader-writer blocking and improves concurrency. However, it introduces issues such as Dirty Reads, Non-Repeatable Reads, Phantom Reads, missing rows, duplicate rows, and inconsistent results during page splits or ongoing updates. It is commonly used through the NOLOCK hint, but NOLOCK does not guarantee accurate data. In modern systems, READ_COMMITTED_SNAPSHOT (RCSI) is generally preferred because it provides non-blocking reads while still returning committed and consistent data.

Enable read uncommitted isolation level

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;

SELECT * FROM Accounts;

Keep in mind: All queries in the session use READ UNCOMMITTED. Other session is still using default isolation level (read committed).

Lock Behavior

Operation	READ COMMITTED	READ UNCOMMITTED
Shared Lock	Yes	No
Reads Uncommitted Data	No	Yes
Blocked by Update	Yes	No
Dirty Read Possible	No	Yes
Non-repeatable Read	Possible	Yes
Phantom Read	Possible	Yes

If we are using READ UNCOMMITTED, we can face the below problem.

Ø  Dirty Reads

Ø  Non-Repeatable Reads

Ø  Phantom Reads

Ø  Missing Rows

Ø  Duplicate Rows

Ø  Reading Corrupted Intermediate State

READ UNCOMMITTED vs RCSI

Feature	READ UNCOMMITTED	RCSI
Dirty Reads	Yes	No
Blocking	Very Low	Very Low
Accuracy	Low	High
Version Store	No	Yes
TempDB Usage	No	Yes
Recommended	Rarely	Frequently

We can use this isolation level on below case

Ø  Dashboard queries

Ø  Monitoring scripts

Ø  Ad-hoc reporting

Ø  Large table scans

Never use on below case

Ø  Banking Transactions

Ø  Financial Reports

Ø  Payroll Calculations

Ø  Inventory Systems

Ø  Order Processing

Ø  Regulatory Reporting

READ UNCOMMITTED is the least restrictive isolation level and allows queries to read uncommitted changes. Because it ignores Exclusive Locks, it can produce several anomalies including Dirty Reads, Non-Repeatable Reads, Phantom Reads, Missing Rows, Duplicate Rows, Incorrect Aggregate Results, and Inconsistent Business Data. These issues make it unsuitable for financial and transactional systems. It is generally used only for monitoring, troubleshooting, or approximate reporting where performance is more important than data accuracy.