Not related with POC of VARIABLE but seeing your patches ...
Wouldn't it be better to use just one syntax and message for what to do ON COMMIT ?
When creating a new variable you use
CREATE VARIABLE ... ON COMMIT DROP | ON TRANSACTION END RESET
On PSQL \dV+ you show
Transactional end action
Maybe all them could be just ON COMMIT
CREATE VARIABLE ... [ON COMMIT {NO ACTION, DROP, RESET}] and \dV+ just "on commit" on title column
ON COMMIT DROP is related to temporary objects. In this case, you don't need to think about ROLLBACK, because in this case, the temp objects are removed implicitly.
ON TRANSACTION END RESET can be used for non temporary objects too. So this is a little bit of a different feature. But the reset is executed if the transaction is ended by ROLLBACK too. So using a syntax just ON COMMIT can be a little bit messy. TRANSACTION END is more intuitive, I think. If I remember there was a proposal ON COMMIT OR ROLLBACK, but I think TRANSACTION END is better and more intuitive, and better describes what is implemented. I can imagine to support clauses ON COMMIT RESET or ON ROLLBACK RESET that can be used independently, but for this time, I don't want to increase a complexity now - reset is just at transaction end without dependency if the transaction was committed or rollbacked.
