INSERT INTO mytable (id, field1, field2) VALUES (val_id, old_val1, old_val2)

UPDATE mytable SET field1=old_val1, field2=old_val2 WHERE id=val_id

sql_select="val_id as id, old_val1 as field1, old_val2 as field2"

sql_text='SELECT aa.id FROM (SELECT ' || sql_select|| ' FROM mytable) AS aa' 
sql_text='SELECT aa.field1 FROM (SELECT ' ||sql_select|| ' FROM mytable) AS aa' 
sql_text='SELECT aa.field2 FROM (SELECT ' ||sql_select|| ' FROM mytable) AS aa' 

sql_text='SELECT aa.id FROM (SELECT ' || sql_select|| ') AS aa' 
sql_text='SELECT aa.field1 FROM (SELECT ' ||sql_select|| ') AS aa' 
sql_text='SELECT aa.field2 FROM (SELECT ' ||sql_select|| ') AS aa' 

SELECT aa.id FROM (SELECT val_id AS id, old_val1 AS field1, old_val2 AS field2) AS aa  

CREATE TABLE my_log
(
    id int4, -- идентификатор 
    creationDate date, -- дата, а лучше и время
    creator int4, -- автор 
    
    table_name varchar( 128 ), -- таблица, в которой прошли изменения    
    rec_id int4, -- запись, в которой прошли изменения
    rec_deleted bool, -- флаг, true-запись удалена, false-изменена
    old_values varchar -- СТАРЫЕ ЗНАЧЕНИЯ одной строкой, например='old_val1 as field1, old_val2 as field2'
)

'old_val1 as field1, old_val2 as field2'

SELECT aa.field1 FROM (SELECT old_val1 AS field1, old_val2 AS field2) AS aa; 
SELECT aa.field2 FROM (SELECT old_val1 AS field1, old_val2 AS field2) AS aa;