If PostgreSQL crashes, there will be a message in the server log with the severity level of PANIC. PostgreSQL will immediately restart and attempt to recover using the transaction log or Write-Ahead Log (WAL).

The WAL consists of a series of files written to the pg_xlog subdirectory of the PostgreSQL data directory. Each change made to the database is recorded first in WAL, hence the name "write-ahead" log, as a synonym of "transaction log". When a transaction commits, the default—and safe—behavior is to force the WAL records to disk. Should PostgreSQL crash, the WAL will be replayed, which returns the database to the point of the last committed transaction, and thus ensures the durability of any database changes.