[
https://issues.apache.org/jira/browse/CASSANDRA-6588?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13874587#comment-13874587
]
Sylvain Lebresne commented on CASSANDRA-6588:
---------------------------------------------
bq. when our existing syntax is up to the task (even if our parser isn't)
I could argue that if your parser don't parse it, then our existing syntax is
not up to the task :P
bq. How big of a pain point is this really?
Quoting myself from CASSANDRA-6586, "This does mean that if you plan on mixing
some (CQL) columns with small values and some with large ones (blobs), and that
you very often need to only query the small values alone, it might be a better
idea to separate small and large values in separate table (or wait for and use
CASSANDRA-6588)". Without saying such use cases are the most common ones, but I
really don't think they are unheard of either (which is confirmed by the
following comment on CASSANDRA-6586), and I think this ticket is order of
magnitude better than my suggestion of "separating small and large values in
separate table", because the latter option could be pretty unintuitive and I
think CQL should avoid getting in the way of modeling things in a relatively
intuitive way.
I'm not saying I'm super thrilled about this option either, but I do
acknowledge that not being able to only query internally a subset of the
columns of a CQL row can have a noticeable performance impact for a non 0
amount of use cases, and that this ticket offers a rather good ratio (and the
best I personally can come up with) of 'providing a solution to this problem'
divided by 'complexity added (to the code and to the CQL syntax)'.
> 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.
--
This message was sent by Atlassian JIRA
(v6.1.5#6160)
