Re: ON CONFLICT and WHERE - Mailing list pgsql-general

On 11/13/22 13:07, Tom Lane wrote:
> Adrian Klaver <adrian.klaver@aklaver.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.klaver@aklaver.com