How to diagnose and fix the 39P02 srf_protocol_violated error code in Postgres.

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The 39P02 srf_protocol_violated error in PostgreSQL is related to set-returning functions (SRFs). This error occurs when a set-returning function (such as a function that returns a set of records or a table) is used improperly, violating the expected protocol for set-returning functions.

A common scenario that leads to this error is when a set-returning function is called in a context that expects a single value, such as in a SELECT list, and the function’s implementation does not follow the correct protocol for returning multiple rows.

Here are some examples and sample code to help diagnose and fix the issue:

1. Incorrect Use of RETURN NEXT in a PL/pgSQL Function

In PL/pgSQL, when writing a set-returning function, you should use RETURN NEXT to return rows one by one, and RETURN QUERY to return the result of a query.

Example of incorrect usage:

CREATE OR REPLACE FUNCTION get_multiple_records_bad()
RETURNS SETOF record AS $$
DECLARE
  rec record;
BEGIN
  FOR rec IN SELECT * FROM my_table LOOP
    RETURN rec; -- Incorrect usage, should be RETURN NEXT
  END LOOP;
END;
$$ LANGUAGE plpgsql;

Fix:
Use RETURN NEXT to return individual rows, or RETURN QUERY to return a query result.

CREATE OR REPLACE FUNCTION get_multiple_records_good()
RETURNS SETOF record AS $$
BEGIN
  RETURN QUERY SELECT * FROM my_table;
END;
$$ LANGUAGE plpgsql;

2. Incorrect Return Type in Function Declaration

The function’s return type must match the actual set of rows being returned. If there is a mismatch, this can lead to the srf_protocol_violated error.

Example of incorrect return type:

CREATE OR REPLACE FUNCTION get_values_bad()
RETURNS integer AS $$
BEGIN
  RETURN QUERY SELECT id, name FROM my_table; -- This returns more than one column
END;
$$ LANGUAGE plpgsql;

Fix:
Ensure that the return type correctly reflects the structure of the returned set.

CREATE OR REPLACE FUNCTION get_values_good()
RETURNS TABLE (id integer, name text) AS $$
BEGIN
  RETURN QUERY SELECT id, name FROM my_table;
END;
$$ LANGUAGE plpgsql;

Diagnosing the Issue

To diagnose the 39P02 srf_protocol_violated error, you should:

  • Check the function definition to ensure that it uses RETURN NEXT or RETURN QUERY appropriately when returning multiple rows.
  • Verify that the return type of the function matches the actual data structure being returned.
  • Ensure that the function is used in a context that expects a set of rows, such as in the FROM clause of a SELECT statement, rather than in places where a single value is expected.

By reviewing the function’s code and its usage, you can identify the cause of the 39P02 srf_protocol_violated error and make the necessary corrections. It is important to follow the correct protocol for set-returning functions to prevent such errors and ensure the expected behavior of your PostgreSQL functions.

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