Specialize 'select-or-insert' and 'update-or-insert'

Recent work on adding the select-or-insert and update-or-insert idioms to EdgeQL (#1628, #684) spawned a discussion on whether using a general-purpose conditional operator ?? is a good idea after all.

The approach committed in #1628 has lots of technical limitations, and, although most of those can be fixed with more work, the main issue with it remains: it gives a misleading appearance of generic support for conditional DML which is impossible as long as it is in PostgreSQL.

For this reason I propose reverting the select-or-insert and update-or-insert to a specialized form of the INSERT statement:

INSERT Type { <shape> } 
UNLESS CONFLICTS [ <conflict-target> [ ELSE <alternative-stmt> ] ]

where alternative-stmt may be any other query, including UPDATE or DELETE.

This is semantically much closer to PostgreSQL’s INSERT ... ON CONFLICT ... and allows explicit specification of the conflict target (i.e. the constraint expression).

[1st1]

I see two different spellings used in this issue and its comments: UNLESS CONFLICTS and UNLESS CONFLICT. I’m +1 for the latter. Also, I think that the optional <conflict-target> should be prefixed by ON.

INSERT Type { <shape> } 
UNLESS CONFLICT [ ON <conflict-target> [ ELSE <alternative-stmt> ] ]

[elprans]

No, DELETE UNLESS CONFLICT would have a drastically different implementation and the use case for it isn’t clear.