What is a Phantom Read and how can it be solved?
Jun 12, 2025 am 10:40 AMPhantom reading refers to the phenomenon of executing the same query twice in a transaction but obtaining different row sets, which are usually caused by inserting or deleting data by another transaction. 1. Use serialized isolation levels to lock the entire data range to prevent phantom reading but may affect performance; 2. Use range locks or key range locks to avoid full table locks and prevent new rows from inserting; 3. Use optimistic concurrency control to detect and deal with phantom reading problems at commit time. This problem is particularly important when multiple queries are required to maintain consistency, such as financial reporting, inventory management and other scenarios.
A phantom read happens in databases when a transaction runs the same query twice and gets different sets of rows. This usually occurs because another transaction inserted or deleted data between the two queries, and those changes are visible to the first transaction. It's a problem when consistency across repeated reads is important.
What Causes Phantom Reads?
Phantom reads typically happen under lower isolation levels like Read Committed , where only committed data is visible, but new rows can still slip in. For example:
- You run a query:
SELECT * FROM orders WHERE status = 'pending';
- Another user inserts a new pending order.
- You run the same query again — now there's an extra row you didn't see before.
This isn't a bug — it's just how some database systems behave by default.
How Is It Different From a Non-Repeatable Read?
It's easy to confuse phantom reads with non-repeatable reads, but they're not the same:
- Non-repeatable read : Same row, different values ??(eg, an updated field).
- Phantom read : New rows appear that match your query criteria.
The distinction matters when choosing how to handle these issues.
How to Prevent Phantom Reads
To stop phantom reads, you need to use a higher isolation level or apply specific locking strategies . Here are the most common solutions:
1. Use Serializable Isolation Level
This is the strictest isolation level and prevents both non-repeatable reads and phantom reads.
- It works by locking the entire range of data being queried.
- Example: If you're querying all pending orders, the database locks the "range" so no new rows can be inserted during your transaction.
Downsides:
- Can cause performance issues due to heavy locking.
- May lead to more deadlocks or slower response times.
2. Use Range Locks or Key-Range Locks
Some databases (like SQL Server) let you lock ranges explicitly.
- This avoids full table locking while still preventing phantom rows.
- It ensures that any insertions into the queried range will wait until the lock is released.
3. Use Optimistic Concurrency Control (OCC)
Instead of locking, OCC checks if data has changed before committing a transaction.
- Works well for low-contention scenarios.
- If a phantom row appears, the system can detect it and retry or abort the transaction.
Use this if locking feels too heavy for your application.
When Should You Care About Phantom Reads?
You should care if your application relies on consistent result sets over multiple queries within the same transaction. Examples include:
- Financial reports
- Inventory management
- Batch processing jobs
In less critical applications — like simple dashboards — phantom reads might not matter much and can be ignored for performance reasons.
In Summary
Phantom reads occur when new rows show up unexpectedly between two identical queries in the same transaction. To prevent them, raise your isolation level to Serializable , use range locks , or apply optimistic concurrency control depending on your needs.
It's not overly complex, but it's something many developers overlook until it causes inconsistencies in production.
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