How to diagnose and fix the 39004 null_value_not_allowed error code in Postgres.

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The 39004 error code in PostgreSQL indicates a situation where a NULL value is not allowed but has been provided. This typically occurs when you try to insert or update a record in a table column that has a NOT NULL constraint, or when a function that is not designed to handle NULL input is given such a value.

Here are some examples of situations where the 39004 error code might occur, along with sample code to diagnose and fix the issue:

  1. Inserting or Updating a Table with NOT NULL Constraints:
    If you attempt to insert or update a record with a NULL value in a column that has a NOT NULL constraint, you will encounter this error. Diagnosis: Check the table definition to see which columns have NOT NULL constraints.
   -- Check the table definition
   \d your_table

Fix: Ensure that your INSERT or UPDATE statement provides non-NULL values for all columns that have NOT NULL constraints.

   -- Insert a record with a non-NULL value in the not-null column
   INSERT INTO your_table (not_null_column, other_column) VALUES ('non-null value', 'other value');

   -- Update a record to ensure the not-null column is not set to NULL
   UPDATE your_table SET not_null_column = 'non-null value' WHERE id = your_id;
  1. Using Functions that Do Not Accept NULL Values:
    If a function is called with NULL arguments but is not designed to handle them, it may raise this error. Diagnosis: Check the function definition to understand its expected parameters and whether it is designed to handle NULL values. Fix: Modify the function to handle NULL values appropriately, or ensure that NULL values are not passed to it.
   -- Modify the function to handle NULL values
   CREATE OR REPLACE FUNCTION your_function(your_parameter your_type)
   RETURNS your_return_type AS $$
   BEGIN
     IF your_parameter IS NULL THEN
       -- Handle the NULL case
     ELSE
       -- Proceed as normal
     END IF;
   END;
   $$ LANGUAGE plpgsql;

Or, when calling the function, make sure to check for NULL values:

   -- Call the function with a non-NULL value
   SELECT your_function(COALESCE(your_potentially_null_value, 'default value'));
  1. Default Values for Table Columns:
    If you are relying on default values for columns, ensure that the defaults are not set to NULL if the column has a NOT NULL constraint. Diagnosis: Check the table definition for default values. Fix: Alter the table to set a proper default value for the column.
   -- Set a default value for the column
   ALTER TABLE your_table ALTER COLUMN not_null_column SET DEFAULT 'default value';
  1. Trigger Functions:
    If a trigger function attempts to set a NULL value in a NOT NULL column, it will result in this error. Diagnosis: Check the trigger function’s code. Fix: Modify the trigger function to ensure it does not attempt to insert or update NULL values into NOT NULL columns.
   -- Modify the trigger function
   CREATE OR REPLACE FUNCTION your_trigger_function()
   RETURNS TRIGGER AS $$
   BEGIN
     NEW.not_null_column := COALESCE(NEW.not_null_column, 'default value');
     RETURN NEW;
   END;
   $$ LANGUAGE plpgsql;

In any case, when encountering the 39004 error code, carefully examine the context in which the error occurred and ensure that any operation involving potential NULL values is handled correctly. For further details on error codes, the PostgreSQL documentation provides a comprehensive list of error codes and their descriptions.

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