Question

Transaction T1 reads a row twice within the same transaction. Between the two reads, T2 commits an update to that row, and T1's second read returns the new value. Which phenomenon does this describe, and which isolation level prevents it?

Q: Answer-Transaction T1 reads a row twice within the same transaction. Between the two reads, T2 commits an u...

A dirty read involves reading uncommitted data. Here T2s change was committed, but T1 saw two DIFFERENT values for the same row within one transaction this is a non-repeatable read, which REPEATABLE READ and SERIALIZABLE isolation levels prevent by holding read locks or using snapshots.