I was the primary author of the feature. That decision wasn't related to hints at all. MERGE just doesn't address the problem that I wanted to address.
I think that you must be referring to Oracle's ignore_row_on_dupkey_index. Is that really in widespread use? While it is technically a hint, it is unlike any other hint it at least one important way: it changes the semantics of the query!
I was thinking of the hints you used to have to use with a merge statement in order to get an atomic upsert - holdlock in sql server and lock table in oracle. My recollection of the terribly old newsgroup thread where it was (presumably you) said merge doesn't perform atomic upserts without these hints and since people were primarily wanting merge to perform atomic upserts and postgres doesn't support hints it made more sense to just make something for upserts hence the on conflict update stuff. Do I have it wrong?
Because of all the potential edge cases that MERGE didn't handle properly when introduced to SQL Server (2008?), I've avoided its use. Despite several revisions over the years there are apparently still significant behaviours that I'd consider bugs¹, some of which have been closed as WONTFIX² so are a fact of life you just have to accept if you use MERGE in SQL Server.
[1] see https://sqlsunday.com/2021/05/04/how-merge-can-deadlock-you/ & https://michaeljswart.com/2021/08/what-to-avoid-if-you-want-... and many other similar articles
[2] https://www.mssqltips.com/sqlservertip/3074/use-caution-with...
"Use the MERGE statement to select rows from one or more sources for update or insertion into a table or view. You can specify conditions to determine whether to update or insert into the target table or view.
This statement is a convenient way to combine multiple operations. It lets you avoid multiple INSERT, UPDATE, and DELETE DML statements."
MERGE doesn't claim to work as an atomic statement, and doesn't work as an atomic statement in practice. In other words you can get a unique violation error where a "true upsert" would update the row instead. You can work around this "limitation" in MERGE by retrying the statement until it succeeds, but at that point you might as well use a regular INSERT instead.
The syntax for INSERT .. ON CONFLICT constrains the problem in certain ways, which is quite deliberate. It's a pretty faithful representation of what's really going on. Hiding that creates a lot of subtle but real problems.
Statement-level atomicity doesn't mean that you cannot get race conditions like the one that I described - it just means that the statement succeeds or fails as a whole.
It's quite clear that you can get those race conditions in both Oracle and SQL Server's MERGE (same with the new Postgres implementation). Last I checked neither SQL Server nor Oracle address the fact that MERGE pretty much doesn't do what many of its users imagine it can do. (There is a narrow theoretical sense in which it does meet expectations provided serializable isolation level is used, but in practice that isn't worth much to users.)
> anyway my experience has been that adding the lock hints "fixes" the problem with pk violations during concurrent upserts and it seemed to be commonly known and widely used afaict
That's not my impression:
https://www.mssqltips.com/sqlservertip/3074/use-caution-with...
So PG doesn't like hints and instead makes non standard syntax for upsert instead of standard merge syntax with a hint.
Now PG is adding merge anyway and probably could have just extended it with non standard syntax to get an atomic upsert instead it has two different approaches.
IMO it probably would have been better to go with the SQL standard MERGE with a non-standard hint syntax so its more readable between systems instead of adding non standard syntax then adding the standard much later.
Not sure if there is any movement on adding the upsert approach to the SQL standard instead.
> PostgreSQL doesn't have a good way to lock access to a key value that doesn't exist yet--what other databases call key range locking (SQL Server for example). Improvements to the index implementation are needed to allow this feature.
as of 2017 at least the plan was to implement merge using the underlying insert on conflict mechanism and I think everyone agrees that the only reasonable expected behavior from merge is atomic upserts (and the pk violations from concurrent merges on sql server and oracle really should be recognized as bugs!) so maybe postgres merge will be the first non-buggy implementation w/o hints. reading the related threads exposes a surprising amount of complexity though so who knows.
That was something that was discussed at length at the time, but ultimately rejected. And so the implementation of MERGE in Postgres has essentially the same limitations as every other MERGE implementation that I'm familiar with.
Postgres clearly stresses the trade-off that MERGE makes in the docs, though. Which probably sets it apart.