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PostgreSQL defaults to Read Committed, which gives each statement a fresh view of committed data. That can suit a simple transfer between two known account rows, but a ledger rule that depends on a changing set of rows or an aggregate may need stronger protection. Repeatable Read gives a transaction a stable snapshot; Serializable additionally detects dangerous concurrent dependencies and may abort a transaction. In either case, application logic must handle failures appropriately.
What transaction isolation means for a ledger
Isolation controls what a transaction can see while other transactions are running, and how PostgreSQL handles concurrent reads and writes. It does not, by itself, define accounting correctness, auditability, durability policy, or regulatory compliance. Those depend on the wider system and its rules.
The key design question is the shape of the invariant. A transaction that updates two predetermined rows differs from one that reads a set of accounts, checks an aggregate or predicate, and then changes a different row. The latter depends on relationships among data that may change concurrently.
How PostgreSQL’s three effective isolation levels differ
| Level | What a transaction sees | Concurrency implications |
|---|---|---|
Read Committed (default) |
Each statement sees data committed before that statement began. | Successive statements can see different committed data. A concurrent update may wait and then operate on the updated row version if it still matches the command’s search condition. |
Repeatable Read |
A stable snapshot established by the transaction’s first non-transaction-control statement, plus the transaction’s own writes. | Other transactions’ later commits remain outside that snapshot. PostgreSQL prevents phantom reads at this level, but serialization anomalies are still possible; conflicting updates may also be aborted. |
Serializable |
The same snapshot foundation as Repeatable Read. | PostgreSQL monitors read/write dependencies and aborts a transaction if needed to preserve an outcome equivalent to a serial execution. |
PostgreSQL treats Read Uncommitted as Read Committed; it does not expose uncommitted writes. The PostgreSQL manual calls Serializable “the strictest transaction isolation” and identifies Read Committed as PostgreSQL’s default. See PostgreSQL 18: Transaction Isolation.
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When Read Committed can fit a known-row transfer
PostgreSQL’s manual uses a transfer between two predetermined account rows as an example of a case that works under Read Committed:
BEGIN;
UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 12345;
UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 7534;
COMMIT;The example relies on each statement targeting a known row and applying its change to the current version of that row. It is a narrow illustration of PostgreSQL’s concurrency behavior, not a universal recommendation for financial systems or a complete ledger design.
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Read Committed can be more difficult when commands use complex search conditions. A statement’s snapshot and concurrent updates can interact in ways that produce an inconsistent view of such changes. Assess the actual rows read and written, and the invariant the transaction must maintain.
What Repeatable Read protects—and what it does not
Repeatable Read keeps a transaction’s view stable from its first non-transaction-control statement. This prevents a later statement in the transaction from seeing another transaction’s newly committed rows or changes, and PostgreSQL’s implementation prevents phantom reads as well.
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A stable snapshot is not the same as a guarantee that concurrent transactions have a serial outcome. For example, a transaction might read several rows or calculate an aggregate, then update a different row based on what it found. Another transaction can make a concurrent decision based on an overlapping view. Repeatable Read alone can permit a serialization anomaly in this kind of dependency pattern.
PostgreSQL cautions that enforcing business rules at this level may require carefully designed explicit locks. Locks can block concurrent work, so their scope and effect need to fit the rule being protected. Repeatable Read can also fail when a transaction tries to modify or lock a row changed since its snapshot began.
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When a ledger should consider Serializable
Serializable is relevant when correctness depends on the combined effect of concurrent reads and writes, rather than only on applying updates to predetermined rows. PostgreSQL tracks read/write dependency patterns that could result in a serialization anomaly; predicate locks help track whether concurrent writes would have affected earlier reads. These predicate locks do not themselves block other transactions.
If PostgreSQL cannot preserve an outcome equivalent to some serial execution, it rolls back a transaction instead of allowing the unsafe combination to commit. That means the application must be prepared to rerun the transaction logic. Serializable adds monitoring and retry overhead, but it is not necessarily slower than explicit locking in every workload: PostgreSQL notes that it can be the best performance choice in some environments. There is no universal best isolation level or workload-independent throughput figure.
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Choose based on the invariant and the failure path
- Known rows, direct changes: Read Committed may suit operations whose statements target predetermined rows, as in PostgreSQL’s documented transfer example.
- Repeated reads of a consistent snapshot: Repeatable Read provides a transaction-level snapshot, but does not eliminate every serialization anomaly.
- Decisions based on related reads, predicates, or aggregates: Evaluate whether Serializable or explicit locking is needed to protect the invariant against concurrent decisions.
- Any level that can abort the transaction: Plan for a complete retry where the failure is transient, and ensure the application can safely rerun the decision logic.
Set the isolation level at the right time
Use SET TRANSACTION ISOLATION LEVEL to set characteristics for the current transaction. PostgreSQL does not allow changing the isolation level after the transaction has executed its first query or data-modification statement. The command’s syntax and timing are documented in PostgreSQL 18: SET TRANSACTION.
BEGIN;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- perform the transaction's reads and writes
COMMIT;Choose the level before issuing application queries or modifications in that transaction. PostgreSQL’s default remains Read Committed unless the transaction or database configuration specifies otherwise.
Handle serialization failures by retrying the whole transaction
PostgreSQL reports relevant serialization failures with SQLSTATE 40001. A retry must rerun the complete transaction, including the application logic that decides which statements and values to use—not merely repeat the final SQL statement. The server cannot safely perform that application-level retry automatically. PostgreSQL’s guidance is in PostgreSQL 17: Serialization Failure Handling.
Deadlocks use SQLSTATE 40P01 and may also call for retry handling. Do not treat every constraint failure the same way: unique or exclusion constraint failures can be persistent errors, so blindly retrying them may not resolve the problem.
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PostgreSQL sequence changes are visible immediately and are not rolled back when a transaction aborts. A gap in sequence-generated identifiers therefore cannot, by itself, establish that every transaction committed in gap-free order. This is a specific property of PostgreSQL sequences, not a general conclusion about accounting records.
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