Re: ON CONFLICT and WHERE

On 11/13/22 13:07, Tom Lane wrote:
> Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com> writes:
>> 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;
>
>> I have not used WHERE with ON CONFLICT myself so it took longer then I
>> care to admit to correct the above to:
>
>> 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;
>
>> The question is why did the first case just ignore the WHERE instead of
>> throwing a syntax error?
>
> A WHERE placed there is an index_predicate attachment to the ON CONFLICT
> clause. It doesn't have any run-time effect other than to allow partial
> indexes to be chosen as arbiter indexes. TFM explains
>
> index_predicate
>
> Used to allow inference of partial unique indexes. Any indexes
> that satisfy the predicate (which need not actually be partial
> indexes) can be inferred.
>
> This strikes me as a bit of a foot-gun. I wonder if we should make
> it safer by insisting that the resolved index be partial when there's
> a WHERE clause here. (This documentation text is about as clear as
> mud, too. What does "inferred" mean here? I think it means "chosen as
> arbiter index", but maybe I misunderstand.)

Alright I see how another use of WHERE comes into play.

I do agree with the clarity of the description, especially after looking
at the example:

"
Insert new distributor if possible; otherwise DO NOTHING. Example
assumes a unique index has been defined that constrains values appearing
in the did column on a subset of rows where the is_active Boolean column
evaluates to true:

-- This statement could infer a partial unique index on "did"
-- with a predicate of "WHERE is_active", but it could also
-- just use a regular unique constraint on "did"
INSERT INTO distributors (did, dname) VALUES (10, 'Conrad International')
ON CONFLICT (did) WHERE is_active DO NOTHING;
"

I honestly cannot figure out what that is saying.

>
> regards, tom lane

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