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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →No. Using an index does not make SQL Server’s NOLOCK hint safe. NOLOCK has the same read behavior as READUNCOMMITTED: it can expose uncommitted changes and return an inconsistent result. An index affects how SQL Server accesses data; it does not change the hint’s isolation behavior.
What an index changes—and what NOLOCK changes
An index is an access path: it can help SQL Server find rows, and an INDEX table hint can direct the optimizer to use specified indexes. NOLOCK, by contrast, controls whether shared data locks protect the read. These are separate behaviors, so forcing or using an index does not make a NOLOCK query consistent. Microsoft documents the distinct hint behaviors in its table hints reference.
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What can go wrong with NOLOCK?
Dirty reads
A query can read changes made by a transaction that has not committed. If that transaction later rolls back, the query may have returned data that never became committed database state.
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Rows can be duplicated or missed
Microsoft also warns that a READUNCOMMITTED or NOLOCK query can see records twice or fail to see records at all. An index does not eliminate these risks; they follow from the read behavior, not from whether the query uses an index.
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A scan can fail during data movement
SQL Server documents error 601 with the message, “Could not continue scan with NOLOCK due to data movement.” Microsoft’s error 601 guidance advises retrying the operation or removing the hint, and identifies READ COMMITTED or SNAPSHOT as alternatives. This is a documented possible failure, not a claim that every NOLOCK scan fails.
NOLOCK does not prevent every kind of blocking
NOLOCK can avoid waits caused by shared data locks, but it does not mean the query never blocks. Queries using the hint still acquire schema-stability (Sch-S) locks during compilation and execution. A concurrent schema-modification (Sch-M) lock can block them. Microsoft describes this behavior in its table hints documentation.
Alternatives when you need fewer reader-writer waits without dirty reads
If the goal is to reduce locking contention while avoiding dirty reads, Microsoft documents READ COMMITTED with READ_COMMITTED_SNAPSHOT enabled, and SNAPSHOT isolation, as options to consider. Both use row versioning for reads rather than permitting dirty reads. The appropriate choice depends on the database’s configuration and the application’s transaction requirements; check the target SQL Server version and settings before changing behavior.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstall| Option | Dirty reads | Read behavior | Scope to evaluate |
|---|---|---|---|
NOLOCK / READUNCOMMITTED |
Permitted | Reads without shared data locks protecting against concurrent changes; results can be inconsistent. | Table hint on the query. |
READ COMMITTED with READ_COMMITTED_SNAPSHOT enabled |
Not permitted | Uses row versioning for reads when the database option is enabled. | Database configuration and READ COMMITTED behavior; verify existing settings. |
| SNAPSHOT | Not permitted | Uses row versioning under SNAPSHOT transaction isolation. | Transaction isolation and database configuration; verify application transaction needs. |
For implementation details and the behavior of isolation levels, consult Microsoft’s SET TRANSACTION ISOLATION LEVEL reference.
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