On 01/14/2014 12:44 AM, Peter Geoghegan wrote:
On Mon, Jan 13, 2014 at 12:58 PM, Heikki Linnakangas
<hlinnakan...@vmware.com> wrote:
Well, even if you don't agree that locking all the conflicting rows for
update is sensible, it's still perfectly sensible to return the rejected
rows to the user. For example, you're inserting N rows, and if some of them
violate a constraint, you still want to insert the non-conflicting rows
instead of rolling back the whole transaction.


Right, but with your approach, can you really be sure that you have
the right rejecting tuple ctid (not reject)? In other words, as you
wait for the exclusion constraint to conclusively indicate that there
is a conflict, minutes may have passed in which time other conflicts
may emerge in earlier unique indexes. Whereas with an approach where
values are locked, you are guaranteed that earlier unique indexes have
no conflicting values. Maintaining that property seems useful, since
we check in a well-defined order, and we're still projecting a ctid.
Unlike when row locking is involved, we can make no assumptions or
generalizations around where conflicts will occur. Although that may
also be a general concern with your approach when row locking, for
multi-master replication use-cases. There may be some value in knowing
it cannot have been earlier unique indexes (and so the existing values
for those unique indexes in the locked row should stay the same -
don't many conflict resolution policies work that way?).

I don't understand what you're saying. Can you give an example?

In the use case I was envisioning above, ie. you insert N rows, and if any of them violate constraint, you still want to insert the non-violating instead of rolling back the whole transaction, you don't care. You don't care what existing rows the new rows conflicted with.

Even if you want to know what you conflicted with, I can't make sense of what you're saying. In the btreelock approach, the value locks are immediately released once you discover that there's conflict. So by the time you get to do anything with the ctid of the existing tuple you conflicted with, new conflicting tuples might've appeared.

- Heikki


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