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