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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-5619) CAS UPDATE for a lost race: save round trip by returning column values
Date Mon, 08 Jul 2013 08:25:48 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-5619?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13701868#comment-13701868
] 

Sylvain Lebresne commented on CASSANDRA-5619:
---------------------------------------------

bq. Could we do timestamp(column) and ttl(column), including literal parentheses, where column
is the name of the column involved?

In the case I'm talking about, there isn't a specific column involved (or rather there could
be more than one). The one I'm talking about is
if you do:
{noformat}
UPDATE foo USING TTL ? SET c1=2, c2=3, ....
{noformat}
In that case the TTL applies to all the columns. And so in that case we currently return [ttl]
(with the square bracket btw) as name for the first prepared marked.

bq. These would not be legal column names

Actually, they would. Provided you double quote a column name, we're pretty permissive about
what it can be: it can be anything really, and "timestamp()" is as much legal than "[timestamp]".

This does is a problem somewhat, don't get me wrong. Even ignoring the timestamp/ttl in prepared
statements, if you do:
{noformat}
SELECT "writeTime(c)", writeTime(c) FROM foo;
{noformat}
we'll return a resultSet where both "column" with the string "writeTime(c)" as identifier,
and the only way to distinguish between the arguably weird column name and the function call
is by position in the result set.

But so far we've somewhat dodged that issue saying that you really have to be looking for
trouble to run into this, and that you still can rely on positions in the result set anyway.
We could probably disambiguate this, though so far I've wondered if it was worth the trouble
really.

Anyway, that's somewhat unrelated to this issue but wanted to be complete.
                
> CAS UPDATE for a lost race: save round trip by returning column values
> ----------------------------------------------------------------------
>
>                 Key: CASSANDRA-5619
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5619
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 2.0 beta 1
>            Reporter: Blair Zajac
>            Assignee: Sylvain Lebresne
>             Fix For: 2.0 beta 1
>
>         Attachments: 0001-Add-back-boolean-result-column.txt, 5619_thrift_fixup.txt,
5619.txt
>
>
> Looking at the new CAS CQL3 support examples [1], if one lost a race for an UPDATE, to
save a round trip to get the current values to decide if you need to perform your work, could
the columns that were used in the IF clause also be returned to the caller?  Maybe the columns
values as part of the SET part could also be returned.
> I don't know if this is generally useful though.
> In the case of creating a new user account with a given username which is the partition
key, if one lost the race to another person creating an account with the same username, it
doesn't matter to the loser what the column values are, just that they lost.
> I'm new to Cassandra, so maybe there's other use cases, such as doing incremental amount
of work on a row.  In pure Java projects I've done while loops around AtomicReference.html#compareAndSet()
until the work was done on the referenced object to handle multiple threads each making forward
progress in updating the references object.
> [1] https://github.com/riptano/cassandra-dtest/blob/master/cql_tests.py#L3044

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