Jepsen: PostgreSQL 12.3

PostgreSQL is a major open-source relational database with a 23-year history and a broad range of features. While Jepsen’s work has traditionally focused on distributed systems, our tooling is readily applicable to traditional, single-node databases. In this report, we present the results of applying Jepsen’s generative concurrency testing to PostgreSQL 12.3.

Prior to version 9.1, PostgreSQL’s documentation claimed to offer up to serializability, “as if transactions had been executed one after another, serially, rather than concurrently…. The Serializable mode provides a rigorous guarantee that each transaction sees a wholly consistent view of the database.” However, this was not true: PostgreSQL’s “serializable” was in fact snapshot isolation (SI).

Informally, snapshot isolated systems appear to start each transaction with a fixed, instantaneous snapshot of the database, reflecting only committed state. Writes performed in a transaction appear to apply atomically at commit time, and a transaction can only commit if no other transaction has modified the same objects as the transaction has written, since the snapshot was taken. This is (as the paper which formalized snapshot isolation made clear) not serializable: transactions whose write sets do not intersect can commit without observing each other’s effects, which could lead to application-level consistency violations.

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