PostgreSQL error 39P01 "Trigger protocol violated" occurs when a trigger function does not return the correct value type or structure. BEFORE triggers must return NULL or a row matching the table structure; AFTER triggers must also return a value. Ensure your trigger function is properly defined with the TRIGGER return type.
Error 39P01 indicates that a trigger function has violated PostgreSQL's trigger protocol by returning an invalid value or having an incorrect function signature. Trigger functions in PostgreSQL must follow strict rules: they must be declared with a TRIGGER return type, and BEFORE row-level triggers must return either NULL (to skip the operation) or NEW/OLD (a row matching the table structure). AFTER triggers should also return a value, though it is typically ignored. When a trigger function fails to meet these requirements, PostgreSQL raises error 39P01 to prevent unpredictable behavior or data corruption.
Check that your trigger function is declared with RETURNS TRIGGER. For example:
CREATE OR REPLACE FUNCTION my_trigger_func()
RETURNS TRIGGER AS $$
BEGIN
-- trigger logic here
END;
$$ LANGUAGE plpgsql;The RETURNS TRIGGER keyword is required for all trigger functions.
BEFORE row-level triggers must return NEW (for INSERT/UPDATE), OLD (for DELETE), or NULL:
CREATE OR REPLACE FUNCTION before_update_func()
RETURNS TRIGGER AS $$
BEGIN
NEW.updated_at = NOW();
RETURN NEW; -- Must return NEW for BEFORE INSERT/UPDATE
END;
$$ LANGUAGE plpgsql;Returning NULL will skip the operation (useful for validation). Returning the row continues the operation with any modified values.
Even though AFTER triggers ignore the return value, you must still include a RETURN statement:
CREATE OR REPLACE FUNCTION after_insert_func()
RETURNS TRIGGER AS $$
BEGIN
INSERT INTO audit_log (action, record_id) VALUES ('INSERT', NEW.id);
RETURN NEW; -- Required even for AFTER triggers
END;
$$ LANGUAGE plpgsql;Ensure the trigger references the function with the correct syntax:
CREATE TRIGGER my_table_before_update
BEFORE UPDATE ON my_table
FOR EACH ROW
EXECUTE FUNCTION before_update_func();For row-level triggers on BEFORE events, use FOR EACH ROW. Statement-level triggers use FOR EACH STATEMENT.
After fixing the function, test it with a basic insert, update, or delete:
INSERT INTO my_table (name) VALUES ('test');If the error persists, check the PostgreSQL server logs for more details:
SELECT pg_current_logfile();View the log file to see the exact error message from the trigger function.
Trigger protocol violations can also occur when using procedural languages other than PL/pgSQL (like PL/Python or PL/Perl), which have different conventions for returning values. If using these languages, ensure the return value is explicitly constructed to match PostgreSQL's trigger protocol. Additionally, some ORMs and database libraries may have issues when AFTER triggers are defined on tables with insert/update operations. If disabling the trigger resolves the issue, the problem is with the trigger function itself, not the application. For complex triggers, consider testing the function in isolation using SELECT to verify its output type and structure before attaching it to the trigger.
ERROR: syntax error at end of input
Syntax error at end of input in PostgreSQL
Bind message supplies N parameters but prepared statement requires M
Bind message supplies N parameters but prepared statement requires M in PostgreSQL
Multidimensional arrays must have sub-arrays with matching dimensions
Multidimensional arrays must have sub-arrays with matching dimensions
ERROR: value too long for type character varying
Value too long for type character varying
insufficient columns in unique constraint for partition key
How to fix "insufficient columns in unique constraint for partition key" in PostgreSQL