On Mon, Sep 30, 2013 at 3:45 PM, Peter Geoghegan <pg@heroku.com> wrote:
> If you think it's a bit odd that we lock every value while the user
> essentially has one constraint in mind when writing their DML,
> consider:
I should add to that list:
4) Locking all the values at once is necessary for the behavior of the
locking to be well-defined -- I feel we need to know that some exact
tuple is to blame (according to our well defined ordering for checking
unique indexes for conflicts) for at least one instant in time.
Given that we need to be the first to change the row without anything
being altered to it, this ought to be sufficient. If you think it's
bad that some other session can come in and insert a tuple that would
have caused us to decide differently (before *our* transaction commits
but *after* we've inserted), now you're into blaming the *wrong* tuple
in the future, and I can't get excited about that - we always prefer a
tuple normally visible to our snapshot, but if forced to (if there is
none) we just throw a serialization failure (where appropriate). So
for read committed you can have no *principled* beef with this, but
for serializable you're going to naturally prefer the
currently-visible tuple generally (that's the only correct behavior
there that won't error - there *better* be something visible).
Besides, the way the user tacitly has to use the feature with one
particular constraint in mind kind of implies that this cannot
happen...
--
Peter Geoghegan