PostgreSQL Fixes

7 troubleshooting guides — each with the error, why it happens, and copy-paste commands to fix it.

• PostgreSQL: "Deadlock Detected" (40P01) — Ordering and the Retry — Two transactions each hold locks the other needs; Postgres detects the cycle and kills one (40P01).
• PostgreSQL "FATAL: role \"postgres\" does not exist" — The role you're connecting as was never created on this cluster — a classic after OS-package installs where the superuser is named after your system user, not 'postgres'.
• PostgreSQL "Connection Refused" on 5432: Cluster Down or Not Listening — Refused on 5432 means nothing is accepting there: cluster not running, listening on localhost only, or wrong port.
• Postgres Down or Slow: Disk Filled by WAL (pg_wal) — Space Recovery Order — When pg_wal bloats to a disk-full, Postgres enters a state where nothing works normally.
• Postgres "the database system is starting up" — Crash Recovery or Stuck? — After an unclean shutdown Postgres replays WAL before accepting connections.
• PostgreSQL "Duplicate Key Value Violates Unique Constraint" — An insert collided with a unique index.
• PostgreSQL: "password authentication failed for user" (and the peer Trap) — The credential failed OR pg_hba.