39P01ERRORTier 2 — Caution✅ HIGH confidence

trigger protocol violated

Category: External Routine Invocation ExceptionVersions: All Postgres versions

What this means

SQLSTATE 39P01 is a Postgres-specific error raised when a trigger function violates the trigger calling protocol — for example, returning an incompatible value from a row-level trigger (must return a RECORD or NULL for row triggers).

Why it happens

1A row-level trigger function returns a value of the wrong type instead of the modified row or NULL
2A trigger function returns a non-RECORD value from a BEFORE row trigger
3PL/C or external language trigger function does not comply with the Postgres trigger calling conventions

How to reproduce

Trigger function returning wrong type.

trigger — this will ERROR

CREATE OR REPLACE FUNCTION bad_trigger() RETURNS TRIGGER AS $
BEGIN
  RETURN 42; -- should return NEW, OLD, or NULL
END;
$ LANGUAGE plpgsql;

ERROR: trigger protocol violated

Fix 1: Return NEW (or OLD) from BEFORE row triggers

When writing PL/pgSQL trigger functions.

fix

CREATE OR REPLACE FUNCTION good_trigger() RETURNS TRIGGER AS $
BEGIN
  -- modify NEW as needed
  RETURN NEW; -- required for BEFORE triggers
END;
$ LANGUAGE plpgsql;

Why this works

BEFORE row triggers must return the row to be inserted/updated (NEW), the original row (OLD), or NULL to cancel the operation. Any other return type violates the protocol.

Sources

📚 Official docs: https://www.postgresql.org/docs/current/errcodes-appendix.html

📚 Feature docs: https://www.postgresql.org/docs/current/plpgsql-trigger.html

🔧 Source ref: Class 39 — External Routine Invocation Exception (Postgres-specific)

Confidence assessment

✅ HIGH confidence

Postgres-specific. Stable across all versions. The trigger return convention is well-documented.