Postgres simple audit trail with trigger

How to implement a simple audit trail with postgres trigger.

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Create the audit_trail table.

CREATE TABLE audit_trail
(
    id           serial PRIMARY KEY,
    time_stamp   timestamp DEFAULT NOW(),
    table_name   text,
    row_pk       int,
    operation    text,
    data         jsonb
);

CREATE INDEX audit_trail_time_stamp_table_name_index
    ON audit_trail (time_stamp DESC, table_name ASC);

When data changes, store only changed data. For comparing new and old data, create a jsonb diff-function.

CREATE FUNCTION jsonb_diff_val(val1 JSONB,val2 JSONB)
RETURNS JSONB AS $$
DECLARE
  result JSONB;
  v RECORD;
BEGIN
   result = val1;
   FOR v IN SELECT * FROM jsonb_each(val2) LOOP
     IF result @> jsonb_build_object(v.key,v.value)
        THEN result = result - v.key;
     ELSIF result ? v.key THEN CONTINUE;
     ELSE
        result = result || jsonb_build_object(v.key,'null');
     END IF;
   END LOOP;
   RETURN result;
END;
$$ LANGUAGE plpgsql;

Create function for logging database changes. On INSERT insert new data, on UPDATE insert changed data, on DELETE insert only id (primary key column).

CREATE FUNCTION log_audit_trail() RETURNS trigger AS
$$
BEGIN
    BEGIN
        IF TG_OP = 'INSERT'
        THEN
            INSERT INTO audit_trail (table_name, row_pk, operation, data)
            VALUES (TG_RELNAME, NEW.id, TG_OP, to_jsonb(NEW));
            RETURN NEW;
        ELSIF TG_OP = 'UPDATE'
        THEN
            INSERT INTO audit_trail (table_name, row_pk, operation, data)
            VALUES (TG_RELNAME, NEW.id, TG_OP, jsonb_diff_val(to_jsonb(NEW), to_jsonb(OLD)));
            RETURN NEW;
        ELSIF TG_OP = 'DELETE'
        THEN
            INSERT INTO audit_trail (table_name, row_pk, operation, data)
            VALUES (TG_RELNAME, OLD.id, TG_OP, to_jsonb(OLD));
            RETURN OLD;
        END IF;
    EXCEPTION
        WHEN OTHERS THEN
            RAISE WARNING 'Error in log_audit_trail function: %', SQLERRM;
            RETURN coalesce(NEW, OLD);
    END;
END;
$$ LANGUAGE 'plpgsql' SECURITY DEFINER;

Create trigger for table, e.g. customer in this example.

CREATE TRIGGER log_customer_changes
    BEFORE INSERT OR UPDATE OR DELETE
    ON "customer"
    FOR EACH ROW
    EXECUTE PROCEDURE log_audit_trail();
Written on March 9, 2023