Database Errors: Emergency Triage & Root Cause Mitigations

Three root causes of FATAL: sorry, too many clients already, single diagnostic query to identify the cause, and permanent mitigation via max_connections and PgBouncer.

Four root causes of psql password authentication failed: incorrect password, md5/SCRAM mismatch, pg_hba.conf rule ordering, and port/cluster confusion. Step-by-step triage.

Four causes of MySQL 1045, meaning of using password: YES/NO, and safe credential recovery via --init-file.

In Error 18456, the State code reveals the actual root cause. Specific fixes for State 5, 8, 11, 18, 38, and 40, with ERRORLOG extraction scripts.

Root causes of MySQL 1213 deadlock, parsing LATEST DETECTED DEADLOCK output, gap lock concurrency issues, and resilient application retry design.

Extracting the deadlock graph from the system_health Extended Events ring buffer, understanding victim election, and architectural resolution.

Difference between connection refused, timeout, and no pg_hba entry; triage for listen_addresses, port conflicts, firewalls, and Docker container mappings.

Difference between connection pool timeout and query command timeout, detecting connection leaks, and .NET DbContext architectural fixes.

Finding the blocking transaction using performance_schema, safe KILL decision matrices, and permanent locking reduction.

Root cause of duplicate key violations after restore or bulk migration, one-line setval repair, and an automated schema-wide PL/pgSQL DO block.

Why replica queries get cancelled due to recovery conflicts, diagnosis via pg_stat_database_conflicts, and three mitigation strategies.

Decoding log_reuse_wait_desc in Error 9002, tailored resolutions for LOG_BACKUP, ACTIVE_TRANSACTION, and AVAILABILITY_REPLICA, plus VLF tuning.

The relationship of reserved connection slots to max_connections, leveraging the superuser recovery window, and tracking down connection leaks.

Distinction between 1452 and 1451, charset and data type mismatch, finding orphan rows, and safe bulk data loading procedures.

Resolving Error 4060 & 18456 State 38/40: deleted default database, offline/restoring status, AUTO_CLOSE traps, and orphan user remediation.

Safely terminating active sessions before DROP DATABASE, ALLOW_CONNECTIONS isolation, prepared transactions, and replication slot blockers.

Distinction between ERROR 28 and ERROR 1114, binlog and undo bloat, inode exhaustion, and non-destructive disk reclamation.

The Docker /dev/shm 64MB default limit, parallel query shared memory exhaustion, proper work_mem calculation, and ordered remediation.

Distinction between Error 1222, 1205 deadlock, and client timeouts, identifying the head blocker session, and RCSI implementation.

Distinguishing ORA-01653, 01654, and 01652, tablespace capacity monitoring, datafile autoextend, and safe sizing with MAXSIZE.