This article is hardly inspired by the original documentation, but I find it a bit hard to read, not really entertaining and answering all the questions. That was the motivation to create more user-friendly version so more people can be familiar with this core concept.

The SQL standard defines four levels of transaction isolation. The most strict is Serializable, which is defined by the standard in a paragraph which says that any concurrent execution of a set of Serializable transactions is guaranteed to produce the same effect as running them one at a time in some order.

The other three levels are defined in terms of phenomena, resulting from interaction between concurrent transactions, which must not occur at each level. The standard notes that due to the definition of Serializable, none of these phenomena are possible at that level.

The phenomena which are prohibited at various levels are:

• dirty read 🤢
  • A transaction reads data written by a concurrent uncommitted transaction.
• non-repeatable read 🔄
  • A transaction re-reads data it has previously read and finds that data has been modified by another transaction (that committed since the initial read).
• phantom read 👻
  • A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that the set of rows satisfying the condition has changed due to another recently-committed transaction.
• serialization anomaly 🤯
  • The result of successfully committing a group of transactions is inconsistent with all possible orderings of running those transactions one at a time.

Transaction Isolation Levels 📊

In PostgreSQL, you can request any of the four standard transaction isolation levels, but internally only three distinct isolation levels are implemented, i.e., PostgreSQL's Read Uncommitted mode behaves like Read Committed.

The table also shows that PostgreSQL's Repeatable Read implementation does not allow phantom reads. This is acceptable under the SQL standard because the standard specifies which anomalies must not occur at certain isolation levels; higher guarantees are acceptable. The behavior of the available isolation levels is detailed in the following subsections.

To set the transaction isolation level of a transaction, use the command SET TRANSACTION.

Read Committed Isolation Level 📖

Read Committed is the default isolation level in PostgreSQL. When a transaction uses this isolation level, a SELECT query (without a FOR UPDATE/SHARE clause) sees only data committed before the query began; it never sees either uncommitted data or changes committed by concurrent transactions during the query's execution.

Read Committed mode starts each command with a new snapshot that includes all transactions committed up to that instant