Re: ON CONFLICT and WHERE

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.)

            regards, tom lane