PostgreSQL raises 39P01 when a trigger function returns or alters data in a way that violates the trigger protocol.
PostgreSQL Error 39P01 – trigger_protocol_violated appears when a trigger function returns an unexpected value or illegally modifies a row. Align the function’s RETURN logic with its timing (BEFORE/AFTER) and level (ROW/STATEMENT); AFTER triggers must RETURN NULL. Fixing the trigger code clears the error.
PostgreSQL Error 39P01
Error 39P01 means that a PostgreSQL trigger function failed to follow the required protocol. The server detected an unexpected return value, row mutation, or context misuse inside the trigger and aborted the command.
The failure usually appears right after an INSERT, UPDATE, DELETE, or TRUNCATE that fires a row-level or statement-level trigger.
PostgreSQL halts the entire transaction to protect data integrity, so correcting the trigger is urgent.
The error occurs when a BEFORE trigger returns NULL where a row is required, returns a row where NULL is required, changes NEW in an AFTER trigger, or produces a tuple with the wrong column count.
Calling a trigger function outside a trigger context also raises 39P01.
Mismatch between trigger timing, level, and the RETURN statement, or copying trigger code across tables without adjustments, frequently leads to protocol violations.
Inspect pg_trigger to confirm timing (BEFORE/AFTER), level (ROW/STATEMENT), and orientation.
Adjust the function so its RETURNS trigger clause and RETURN statement match the definition.
- AFTER triggers: always RETURN NULL.
- BEFORE INSERT/UPDATE row-level triggers: RETURN NEW (or modified NEW) unless you wish to skip the row with RETURN NULL.
- BEFORE DELETE row-level triggers: RETURN OLD or NULL to skip.
Scenario: AFTER UPDATE row-level trigger returns NEW.
Solution: change the final statement to RETURN NULL;.
Scenario: BEFORE INSERT trigger adds audit fields but returns a record with extra columns.
Solution: ensure the returned record exactly matches the target table definition.
Scenario: Statement-level trigger mistakenly written as row-level.
Solution: drop and recreate the trigger with FOR EACH STATEMENT or revise the function.
Unit-test every trigger with representative DML statements.
Place RETURN comments (e.g., -- AFTER triggers must RETURN NULL) to document intent. Keep separate functions for BEFORE and AFTER triggers to limit branching complexity.
After any schema change, recompile and test triggers. Use PostgreSQL’s pg_trigger.tgrelid metadata to audit that each trigger function matches its table’s current structure.
2BP01 dependent_objects_still_exist - raised when dropping a trigger that other objects depend on.
Drop or alter dependents first.
42883 undefined_function - appears if the trigger function was renamed or dropped. Recreate or correct the trigger definition.
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Returning NEW (or a modified row) in an AFTER trigger violates the protocol, because row changes are disallowed after the DML finishes.
A BEFORE INSERT or UPDATE trigger that returns a tuple with extra or missing columns causes 39P01.
FOR EACH STATEMENT triggers must return NULL.
Returning a record triggers the error.
Invoking the trigger function directly in SQL lacks TG_* variables, immediately raising trigger_protocol_violated.
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No. PostgreSQL aborts the transaction before committing, so on-disk data stays consistent.
No. PostgreSQL uses the returned row (or NULL) to decide whether to insert, update, or skip the row.
The UPDATE branch probably returns NEW. AFTER triggers must return NULL regardless of operation type.
Galaxy’s live linting flags mismatched RETURN clauses and trigger orientations as you type, preventing 39P01 before code reaches production.
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