ON CONFLICT and WHERE

In process of answering an SO question I ran across the below.

The original question example:

CREATE TABLE books (
id int4 NOT NULL,
version int8 NOT NULL,
updated timestamp NULL,
CONSTRAINT books_pkey PRIMARY KEY (id)
);

INSERT INTO books VALUES (12, 0, CURRENT_TIMESTAMP)
ON CONFLICT (id)
WHERE version IS NULL OR updated + INTERVAL '2min' < CURRENT_TIMESTAMP
DO UPDATE
SET version = books.version + 1, updated = CURRENT_TIMESTAMP;

select *, CURRENT_TIMESTAMP, updated + INTERVAL '2min' <
CURRENT_TIMESTAMP from books where id = 12;

INSERT INTO books VALUES (12, 0, CURRENT_TIMESTAMP)
ON CONFLICT (id)
WHERE updated IS NULL OR updated + INTERVAL '2min' < CURRENT_TIMESTAMP
DO UPDATE
SET version = books.version + 1, updated = CURRENT_TIMESTAMP;

select *, CURRENT_TIMESTAMP, updated + INTERVAL '2min' <
CURRENT_TIMESTAMP from books where id = 12;

With select results as:

id | version | updated | current_timestamp
| ?column?
----+---------+----------------------------+--------------------------------+----------
12 | 0 | 11/13/2022 12:21:38.032578 | 11/13/2022 12:21:38.057545
PST | f

id | version | updated | current_timestamp
| ?column?
----+---------+----------------------------+--------------------------------+----------
12 | 1 | 11/13/2022 12:21:38.058673 | 11/13/2022 12:21:40.686231
PST | f

I have not used WHERE with ON CONFLICT myself so it took longer then I
care to admit to correct the above to:

DROP TABLE IF EXISTS books;

CREATE TABLE books (
id int4 NOT NULL,
version int8 NOT NULL,
updated timestamp NULL,
CONSTRAINT books_pkey PRIMARY KEY (id)
);

INSERT INTO books VALUES (12, 0, CURRENT_TIMESTAMP)
ON CONFLICT (id)
DO UPDATE
SET version = books.version + 1, updated = CURRENT_TIMESTAMP
WHERE books.version IS NULL OR books.updated + INTERVAL '2min' <
CURRENT_TIMESTAMP;

select *, CURRENT_TIMESTAMP, updated + INTERVAL '2min' <
CURRENT_TIMESTAMP from books where id = 12;

INSERT INTO books VALUES (12, 0, CURRENT_TIMESTAMP)
ON CONFLICT (id)
DO UPDATE
SET version = books.version + 1, updated = CURRENT_TIMESTAMP
WHERE books.version IS NULL OR books.updated + INTERVAL '2min' <
CURRENT_TIMESTAMP;

select *, CURRENT_TIMESTAMP, updated + INTERVAL '2min' <
CURRENT_TIMESTAMP from books where id = 12

With select results as:

id | version | updated | current_timestamp
| ?column?
----+---------+----------------------------+--------------------------------+----------
12 | 0 | 11/13/2022 12:32:01.427769 | 11/13/2022 12:32:01.463705
PST | f

id | version | updated | current_timestamp
| ?column?
----+---------+----------------------------+--------------------------------+----------
12 | 0 | 11/13/2022 12:32:01.427769 | 11/13/2022 12:32:01.476484
PST | f

I ran this on both version 14 and 15 with same results.

The question is why did the first case just ignore the WHERE instead of
throwing a syntax error?

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com