Category
isolation levels
7 articles
Dirty Write Explained: When Uncommitted Data Gets Overwritten
TLDR: A dirty write occurs when Transaction B overwrites data that Transaction A has written but not yet committed. The result is not a rollback or an error — it is silently inconsistent committed dat
Read Skew Explained: Inconsistent Snapshots Across Multiple Objects
TLDR: Read skew occurs when a transaction reads two logically related objects at different points in time — one before and one after a concurrent transaction commits — producing a view that never exis
Lost Update Explained: When Two Writes Become One
TLDR: A lost update occurs when two concurrent read-modify-write transactions both read the same committed value, both compute a new value from it, and both write back — with the second write silently
Phantom Read Explained: When New Rows Appear Mid-Transaction
TLDR: A phantom read occurs when a transaction runs the same range query twice and gets a different set of rows — because a concurrent transaction inserted or deleted matching rows and committed in between. Row locks cannot stop this because the phan...
Write Skew Explained: The Anomaly That Requires Serializable Isolation
TLDR: Write skew is the hardest concurrency anomaly to reason about: two concurrent transactions each read a shared condition, decide they can safely proceed, and then write to different rows. No individual operation is wrong. No row was overwritten....
Dirty Read Explained: How Uncommitted Data Corrupts Transactions
TLDR: A dirty read occurs when Transaction B reads data written by Transaction A before A has committed. If A rolls back, B has made decisions on data that — from the database's perspective — never existed. Read Committed isolation (the default in Po...
Non-Repeatable Read Explained: When the Same Query Returns Different Results
TLDR: A non-repeatable read happens when the same SELECT returns different results within a single transaction because a concurrent transaction committed an update between the two reads. Read Committed isolation — the default in PostgreSQL, MySQL, an...