Within a trigger function, how to get which fields are being updated

In plpgsql you could do something like this in your trigger function:

IF NEW.column IS NULL THEN
  NEW.column = 'default value';
END IF;

If a "source" doesn't "send an identifier", the column will be unchanged. Then you cannot detect whether the current UPDATE was done by the same source as the last one or by a source that did not change the column at all. In other words: this does not work properly.

If the "source" is identifiable by any session information function, you can work with that. Like:

NEW.column = session_user;

Unconditionally for every update.

General Solution

I found a way how to solve the original problem. The column will be set to a default value in any update where the column is not updated (not in the SET list of the UPDATE).

Key element is a per-column trigger introduced in PostgreSQL 9.0 - a column-specific trigger using the UPDATE OF column_name clause.

The trigger will only fire if at least one of the listed columns is mentioned as a target of the UPDATE command.

That's the only simple way I found to distinguish whether a column was updated with a new value identical to the old, versus not updated at all.

One could also parse the text returned by current_query(). But that seems tricky and unreliable.

Trigger functions

I assume a column source defined NOT NULL.

Step 1: Set source to NULL if unchanged:

CREATE OR REPLACE FUNCTION trg_tbl_upbef_step1()
  RETURNS trigger
  LANGUAGE plpgsql AS
$func$
BEGIN
   IF NEW.source = OLD.source THEN
      NEW.source := NULL;      -- "impossible" value (source is NOT NULL)
   END IF;

   RETURN NEW;
END
$func$;

Step 2: Revert to old value. Trigger will only be fired, if the value was actually updated (see below):

CREATE OR REPLACE FUNCTION trg_tbl_upbef_step2()
  RETURNS trigger
  LANGUAGE plpgsql AS
$func$
BEGIN
   IF NEW.source IS NULL THEN
      NEW.source := OLD.source;
   END IF;

   RETURN NEW;
END
$func$;

Step 3: Now we can identify the lacking update and set a default value instead:

CREATE OR REPLACE FUNCTION trg_tbl_upbef_step3()
  RETURNS trigger
  LANGUAGE plpgsql AS
$func$
BEGIN
   IF NEW.source IS NULL THEN
      NEW.source := 'UPDATE default source';  -- optionally same as column default
   END IF;

   RETURN NEW;
END
$func$;

Triggers

The trigger for Step 2 is fired per column!

CREATE TRIGGER upbef_step1
  BEFORE UPDATE ON tbl
  FOR EACH ROW
  EXECUTE PROCEDURE trg_tbl_upbef_step1();

CREATE TRIGGER upbef_step2
  BEFORE UPDATE OF source ON tbl             -- key element!
  FOR EACH ROW
  EXECUTE PROCEDURE trg_tbl_upbef_step2();
    
CREATE TRIGGER upbef_step3
  BEFORE UPDATE ON tbl
  FOR EACH ROW
  EXECUTE PROCEDURE trg_tbl_upbef_step3();

db<>fiddle here

Trigger names are relevant, because they are fired in alphabetical order (all being BEFORE UPDATE)!

The procedure could be simplified with something like "per-not-column triggers" or any other way to check the target-list of an UPDATE in a trigger. But I see no handle for this, currently (unchanged as of Postgres 14).

If source can be NULL, use any other "impossible" intermediate value and check for NULL additionally in trigger function 1:

IF OLD.source IS NOT DISTINCT FROM NEW.source THEN
    NEW.source := '#impossible_value#';
END IF;

Adapt the rest accordingly.

Another way is to exploit JSON/JSONB functions that come in recent versions of PostgreSQL. It has the advantage of working both with anything that can be converted to a JSON object (rows or any other structured data), and you don't even need to know the record type.

To find the differences between any two rows/records, you can use this little hack:

SELECT pre.key AS columname, pre.value AS prevalue, post.value AS postvalue
FROM jsonb_each(to_jsonb(OLD)) AS pre
CROSS JOIN jsonb_each(to_jsonb(NEW)) AS post
WHERE pre.key = post.key AND pre.value IS DISTINCT FROM post.value

Where OLD and NEW are the built-in records found in trigger functions representing the pre and after state respectively of the changed record. Note that I have used the table aliases pre and post instead of old and new to avoid collision with the OLD and NEW built-in objects. Note also the use of IS DISTINCT FROM instead of a simple != or <> to handle NULL values appropriately.

Of course, this will also work with any ROW constructor such as ROW(1,2,3,...) or its short-hand (1,2,3,...). It will also work with any two JSONB objects that have the same keys.

For example, consider an example with two rows (already converted to JSONB for the purposes of the example):

SELECT pre.key AS columname, pre.value AS prevalue, post.value AS postvalue
FROM jsonb_each('{"col1": "same", "col2": "prediff", "col3": 1, "col4": false}') AS pre
CROSS JOIN jsonb_each('{"col1": "same", "col2": "postdiff", "col3": 1, "col4": true}') AS post
WHERE pre.key = post.key AND pre.value IS DISTINCT FROM post.value

The query will show the columns that have changed values:

 columname | prevalue  | postvalue
-----------+-----------+------------
 col2      | "prediff" | "postdiff"
 col4      | false     | true

The cool thing about this approach is that it is trivial to filter by column. For example, imagine you ONLY want to detect changes in columns col1 and col2:

SELECT pre.key AS columname, pre.value AS prevalue, post.value AS postvalue
FROM jsonb_each('{"col1": "same", "col2": "prediff", "col3": 1, "col4": false}') AS pre
CROSS JOIN jsonb_each('{"col1": "same", "col2": "postdiff", "col3": 1, "col4": true}') AS post
WHERE pre.key = post.key AND pre.value IS DISTINCT FROM post.value
AND pre.key IN ('col1', 'col2')

The new results will exclude col3 from the results even if it's value has changed:

 columname | prevalue  | postvalue
-----------+-----------+------------
 col2      | "prediff" | "postdiff"

It is easy to see how this approach can be extended in many ways. For example, say you want to throw an exception if certain columns are updated. You can achieve this with a universal trigger function, that is, one that can be applied to any/all tables, without having to know the table type:

CREATE OR REPLACE FUNCTION yourschema.yourtriggerfunction()
RETURNS TRIGGER AS
$$
DECLARE
    immutable_cols TEXT[] := ARRAY['createdon', 'createdby'];
BEGIN

    IF TG_OP = 'UPDATE' AND EXISTS(
        SELECT 1
        FROM jsonb_each(to_jsonb(OLD)) AS pre, jsonb_each(to_jsonb(NEW)) AS post
        WHERE pre.key = post.key AND pre.value IS DISTINCT FROM post.value
        AND pre.key = ANY(immutable_cols)
    ) THEN
        RAISE EXCEPTION 'Error 12345 updating table %.%. Cannot alter these immutable cols: %.',
            TG_TABLE_SCHEMA, TG_TABLE_NAME, immutable_cols;
    END IF;

END
$$
LANGUAGE plpgsql VOLATILE

You would then register the above trigger function to any and all tables you want to control via:

CREATE TRIGGER yourtiggername
BEFORE UPDATE ON yourschema.yourtable
FOR EACH ROW EXECUTE PROCEDURE yourschema.yourtriggerfunction();