In PL/pgSQL, the RETURN NEXT statement is used to return a row from a function or procedure that is declared as returning a set of rows. However, if the function or procedure is not declared as returning a set of rows, using RETURN NEXT will result in an error.
RETURN NEXT in a function or procedure that is not declared as returning a set of rows. To fix this error, you need to declare the function or procedure as returning a set of rows by using the SETOF keyword in the RETURNS clause.
For example, the following function is declared as returning a set of rows using the SETOF keyword:
```
CREATE FUNCTION get_products() RETURNS SETOF product AS $$
BEGIN
RETURN QUERY SELECT * FROM products;
END;
$$ LANGUAGE plpgsql;
```
Now, you can use the RETURN NEXT statement in this function to return rows from the products table. For example, the following code uses a FOR loop to iterate over the rows returned by the get_products() function and print each row's product name:
```
DO $$
DECLARE
row product;
BEGIN
FOR row IN SELECT * FROM get_products() LOOP
RAISE NOTICE '%', row.product_name;
END LOOP;
END;
$$;
```
When you execute the above code, it will print the product names from the products table.
Another way to fix the "cannot use RETURN NEXT in a non-SETOF function" error is to use the RETURN QUERY statement instead of RETURN NEXT. The RETURN QUERY statement can be used to return a set of rows from a function or procedure, even if the function or procedure is not declared as returning a set of rows.
For example, the following function uses the RETURN QUERY statement to return a set of rows from the products table:
```
CREATE FUNCTION get_products() RETURNS TABLE (product_name text) AS $$
BEGIN
RETURN QUERY SELECT product_name FROM products;
END;
$$ LANGUAGE plpgsql;
```
Now, you can use the RETURN QUERY statement in this function to return rows from the products table. For example, the following code uses a FOR loop to iterate over the rows returned by the get_products() function and print each row's product name:
```
DO $$
DECLARE
row product;
BEGIN
FOR row IN SELECT * FROM get_products() LOOP
RAISE NOTICE '%', row.product_name;
END LOOP;
END;
$$;
```
When you execute the above code, it will print the product names from the products table.