If there's some code which says "if there isn't an EXXON then create
one", and there's a uniqueness constraint such that there can't be two
Bxs, then "clearly" the above code can go wrong, in that the following
cannot succeed:

(1) Transaction 1 - check for EXXON, find it doesn't exist
(2) Transaction 1 - create EXXON
(3) Transaction 2 - check for EXXON, find it doesn't exist (because it
can't see the one created by transaction 1)
(4) Transaction 2 - create EXXON
(5) Transaction 1 - commit
(6) Transaction 2 - commit

This fails, as one would expect, due to the violation of the uniqueness
constraint. But my question is: does it fail at point (4), because the
uniqueness constraint is somehow active/visible/whatever across
transactions, or does it fail at point (6), because the uniqueness
constraint only takes account of committed stuff?

(Yes I do know that's what generators are for.)

--
Tim Ward