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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-4284) Improve timeuuid <-> date relationship
Date Thu, 12 Jul 2012 06:17:34 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-4284?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13412546#comment-13412546
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Sylvain Lebresne commented on CASSANDRA-4284:
---------------------------------------------

bq. It seems like people using timeuuid will "usually" want to see the date value associated
with their timeuuids

I definitively agree on the principle that timeuuid are more dates than they are uuid, at
least in the intention. However, using only the date as the representation means that it's
not really a representation but an estimate.  Concretely, it means that for newcomers:
* it will look like timeuuid breaks the PK constraint (that's just a illusion of the imprecise
representation but still confusing)
* if someone do a read, and reuse what you get to update the row, you won't modify what you'd
expected to modify. I think that one is a big catch because it makes it easy to corrupt the
DB (it's easy to forget the representation is not precise even if you know it).
All this make me worried that showing just the date par by default will be more confusing
than helpful.

bq. I'd much rather have the % syntax addition than to do this

For the reasons above (timeuuid are fundamentally dates, but dates without syntax extension
is dangerous) I think I would prefer that too.

bq. (a) filled in random bits for the MAC address area of the uuids

Don't that have the problem I described above for queries? I.e. if you introduce random by
default in the translation of dates to timeuuid, the same request executed twice could return
different results.

bq. (b) extended the recognized timestamp syntax to allow specifying fractional seconds?

I'm not sure I follow, could you give an example.

                
> Improve timeuuid <-> date relationship
> --------------------------------------
>
>                 Key: CASSANDRA-4284
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-4284
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: API
>    Affects Versions: 1.1.0
>            Reporter: Sylvain Lebresne
>            Assignee: Sylvain Lebresne
>            Priority: Minor
>              Labels: cql3
>             Fix For: 1.2
>
>
> We added timeuuid to CQL3, whose purpose is to provide a collision-free timestamp basically
and as a convenience, such timeuuid can be inputed as as date.
> However, two things seems non-optimal to me:
> * When one insert a timeuuid using a date format, we always pick *the* UUID corresponding
to this date with every other part of the UUID to 0. This kind of defeat the purpose of collision-free
timestamp and thus greatly limit the usefulness of the date syntax.
> * When cqlsh print timeuuid, it print them as date. But as thus, there is some information
lost which can be problematic (you can't update a specific column based on that return). In
a way, this is a cqlsh limitation, since cassandra return the UUID bytes. Yet, it also emphasis
somehow that from the point of using them, timeuuid are more UUID than really time.
> For the first point, it would make more sense that when inserting a date, we actually
pick a uuid with the corresponding timestamp *but* with the rest of the UUID being random.
It's not completely that simple because we don't want that randomness when the date are used
in a select query, but that's roughtly the same problem than CASSANDRA-4283 (and we can thus
use the same solution).
> The second point gives an idea. We could extends the date syntax to allow it to represent
uniquely a type 1 UUID. Typically, we could allow something like: '2012-06-06 12:03:00+0000
%a2fc07', where the part after the '%' character would be hexadecimal for the non-timestamp
part of the UUID. Understanding this syntax could allow to work with timeuuid uniquely with
meaningful date string which I think would be neat. But maybe that's a crazy idea, opinions?

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