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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6588) Add a 'NO EMPTY RESULTS' filter to SELECT
Date Fri, 17 Jan 2014 18:08:22 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6588?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13875033#comment-13875033
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Sylvain Lebresne commented on CASSANDRA-6588:
---------------------------------------------

I'm a rather strong -1 on removing per-column TTL. I've personally used them (I do mean in
cases where per-row would be too coarse grained), I've witnessed at least a few other uses
over the years and per-column TTLs have been here for a long enough that I have no doubt the
cases I've personally witness are not the only exceptions. I'm not saying everyone that uses
TTL needs per-column ones, but there *is* people using per-column TTL, which means this would
be a breaking change that would break some people. The bar to do such breaking changes should
be high: it should be very clear that the benefits of doing such change strongly overweight
the pain inflicted to users we break.  This issue is nowhere near close to meeting that bar
imo.

Even if we were to ignore the breaking aspects, since there is cases where you do want to
put TTL only on some of the columns of a row, if we remove per-column TTL, it means users
will be forced to split such rows into multiple table to be able to apply different TTL on
some columns and not others. And avoiding the need to split table in two due to limitations
of CQL rows is exactly the reason why this ticket exists. Removing per-column TTL to solve
this wouldn't be progress.

Last but not necessary least, I'm far from convinced that per-row TTL would actually solve
this. I suggested on CASSANDRA-5762 that this *might* help but truth is, the vague intuition
I had in mind back then doesn't work at all. I personally don't see how per-row TTL solves
this problem concretely.

Back on the proposition of this ticket, I continue to think that it's a very simple solution,
and while I can understand not being "thrilled" on a personal level, I have a hard time imagining
this confusing people immensely, so I don't a big concrete downside to it (and for what is
worth, at least one user on CASSANDRA-6586, someone that does care about the problem we're
trying to solve here, judged this solution as "promising", which at least suggest that it
might not be so bad).

> Add a 'NO EMPTY RESULTS' filter to SELECT
> -----------------------------------------
>
>                 Key: CASSANDRA-6588
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6588
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Sylvain Lebresne
>            Priority: Minor
>             Fix For: 2.1
>
>
> It is the semantic of CQL that a (CQL) row exists as long as it has one non-null column
(including the PK columns, which, given that no PK columns can be null, means that it's enough
to have the PK set for a row to exist). This does means that the result to
> {noformat}
> CREATE TABLE test (k int PRIMARY KEY, v1 int, v2 int);
> INSERT INTO test(k, v1) VALUES (0, 4);
> SELECT v2 FROM test;
> {noformat}
> must be (and is)
> {noformat}
>  v2
> ------
>  null
> {noformat}
> That fact does mean however that when we only select a few columns of a row, we still
need to find out rows that exist but have no values for the selected columns. Long story short,
given how the storage engine works, this means we need to query full (CQL) rows even when
only some of the columns are selected because that's the only way to distinguish between "the
row exists but have no value for the selected columns" and "the row doesn't exist". I'll note
in particular that, due to CASSANDRA-5762, we can't unfortunately rely on the row marker to
optimize that out.
> Now, when you selects only a subsets of the columns of a row, there is many cases where
you don't care about rows that exists but have no value for the columns you requested and
are happy to filter those out. So, for those cases, we could provided a new SELECT filter.
Outside the potential convenience (not having to filter empty results client side), one interesting
part is that when this filter is provided, we could optimize a bit by only querying the columns
selected, since we wouldn't need to return rows that exists but have no values for the selected
columns.
> For the exact syntax, there is probably a bunch of options. For instance:
> * {{SELECT NON EMPTY(v2, v3) FROM test}}: the vague rational for putting it in the SELECT
part is that such filter is kind of in the spirit to DISTINCT.  Possibly a bit ugly outside
of that.
> * {{SELECT v2, v3 FROM test NO EMPTY RESULTS}} or {{SELECT v2, v3 FROM test NO EMPTY
ROWS}} or {{SELECT v2, v3 FROM test NO EMPTY}}: the last one is shorter but maybe a bit less
explicit. As for {{RESULTS}} versus {{ROWS}}, the only small object to {{NO EMPTY ROWS}} could
be that it might suggest it is filtering non existing rows (I mean, the fact we never ever
return non existing rows should hint that it's not what it does but well...) while we're just
filtering empty "resultSet rows".
> Of course, if there is a pre-existing SQL syntax for that, it's even better, though a
very quick search didn't turn anything. Other suggestions welcome too.



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