Cannot update existing row on conflict in PostgreSQL with Psycopg2 - python-3.x

Question

I have the following function defined to insert several rows with iteration in Python using Psycopg2 and PostgreSQL 11.
When I receive the same obj (with same id), I want to update its date.
def insert_execute_values_iterator(
connection,
objs: Iterator[Dict[str, Any]],
page_size: int = 1000,
) -> None:
with connection.cursor() as cursor:
try:
psycopg2.extras.execute_values(cursor, """
INSERT INTO objs(\
id,\
date,\
) VALUES %s \
ON CONFLICT (id) \
DO UPDATE SET (date) = (EXCLUDED.date) \
""", ((
obj['id'],
obj['date'],
) for obj in objs), page_size=page_size)
except (Exception, Error) as error:
print("Error while inserting as in database", error)
When a conflict happens on the unique primary key of the table while inserting an element, I get the error:
Error while inserting as in database ON CONFLICT DO UPDATE command
cannot affect row a second time
HINT: Ensure that no rows proposed for insertion within the same command have duplicate constrained values.
FYI, the clause works on PostgreSQL directly but not from the Python code.

Answer 1

Use unique VALUE-combinations in your INSERT statement:
create table foo(id int primary key, date date);
This should work:
INSERT INTO foo(id, date)
VALUES(1,'2021-02-17')
ON CONFLICT(id)
DO UPDATE SET date = excluded.date;
This one won't:
INSERT INTO foo(id, date)
VALUES(1,'2021-02-17') , (1, '2021-02-16') -- 2 conflicting rows
ON CONFLICT(id)
DO UPDATE SET date = excluded.date;
DEMO
You can fix this by using DISTINCT ON() in a SELECT statement:
INSERT INTO foo(id, date)
SELECT DISTINCT ON(id) id, date
FROM (VALUES(1,CAST('2021-02-17' AS date)) , (1, '2021-02-16')) s(id, date)
ORDER BY id, date ASC
ON CONFLICT(id)
DO UPDATE SET date = excluded.date;