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From "DOAN DuyHai (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6561) Static columns in CQL3
Date Thu, 20 Feb 2014 20:27:21 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6561?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13907450#comment-13907450
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DOAN DuyHai commented on CASSANDRA-6561:
----------------------------------------

Really great change ! It brings lots of new possibilities for data modeling with CQL3, big
+1.

 A very dumb question though: is it allowed to create a static column in a non-clustered table
?

 I mean, it clearly does not make sense to do so but the question is more on validation &
checking side ? Does C* raise exception if I try to create a static column in a plain non-clustered
table ?

> Static columns in CQL3
> ----------------------
>
>                 Key: CASSANDRA-6561
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6561
>             Project: Cassandra
>          Issue Type: New Feature
>            Reporter: Sylvain Lebresne
>            Assignee: Sylvain Lebresne
>             Fix For: 2.0.6
>
>
> I'd like to suggest the following idea for adding "static" columns to CQL3.  I'll note
that the basic idea has been suggested by jhalliday on irc but the rest of the details are
mine and I should be blamed for anything stupid in what follows.
> Let me start with a rational: there is 2 main family of CF that have been historically
used in Thrift: static ones and dynamic ones. CQL3 handles both family through the presence
or not of clustering columns. There is however some cases where mixing both behavior has its
use. I like to think of those use cases as 3 broad category:
> # to denormalize small amounts of not-entirely-static data in otherwise static entities.
It's say "tags" for a product or "custom properties" in a user profile. This is why we've
added CQL3 collections. Importantly, this is the *only* use case for which collections are
meant (which doesn't diminishes their usefulness imo, and I wouldn't disagree that we've maybe
not communicated this too well).
> # to optimize fetching both a static entity and related dynamic ones. Say you have blog
posts, and each post has associated comments (chronologically ordered). *And* say that a very
common query is "fetch a post and its 50 last comments". In that case, it *might* be beneficial
to store a blog post (static entity) in the same underlying CF than it's comments for performance
reason.  So that "fetch a post and it's 50 last comments" is just one slice internally.
> # you want to CAS rows of a dynamic partition based on some partition condition. This
is the same use case than why CASSANDRA-5633 exists for.
> As said above, 1) is already covered by collections, but 2) and 3) are not (and
> I strongly believe collections are not the right fit, API wise, for those).
> Also, note that I don't want to underestimate the usefulness of 2). In most cases, using
a separate table for the blog posts and the comments is The Right Solution, and trying to
do 2) is premature optimisation. Yet, when used properly, that kind of optimisation can make
a difference, so I think having a relatively native solution for it in CQL3 could make sense.
> Regarding 3), though CASSANDRA-5633 would provide one solution for it, I have the feeling
that static columns actually are a more natural approach (in term of API). That's arguably
more of a personal opinion/feeling though.
> So long story short, CQL3 lacks a way to mix both some "static" and "dynamic" rows in
the same partition of the same CQL3 table, and I think such a tool could have it's use.
> The proposal is thus to allow "static" columns. Static columns would only make sense
in table with clustering columns (the "dynamic" ones). A static column value would be static
to the partition (all rows of the partition would share the value for such column). The syntax
would just be:
> {noformat}
> CREATE TABLE t (
>   k text,
>   s text static,
>   i int,
>   v text,
>   PRIMARY KEY (k, i)
> )
> {noformat}
> then you'd get:
> {noformat}
> INSERT INTO t(k, s, i, v) VALUES ("k0", "I'm shared",       0, "foo");
> INSERT INTO t(k, s, i, v) VALUES ("k0", "I'm still shared", 1, "bar");
> SELECT * FROM t;
>  k |                  s | i |    v
> ------------------------------------
> k0 | "I'm still shared" | 0 | "bar"
> k0 | "I'm still shared" | 1 | "foo"
> {noformat}
> There would be a few semantic details to decide on regarding deletions, ttl, etc. but
let's see if we agree it's a good idea first before ironing those out.
> One last point is the implementation. Though I do think this idea has merits, it's definitively
not useful enough to justify rewriting the storage engine for it. But I think we can support
this relatively easily (emphasis on "relatively" :)), which is probably the main reason why
I like the approach.
> Namely, internally, we can store static columns as cells whose clustering column values
are empty. So in terms of cells, the partition of my example would look like:
> {noformat}
> "k0" : [
>   (:"s" -> "I'm still shared"), // the static column
>   (0:"" -> "")                  // row marker
>   (0:"v" -> "bar")
>   (1:"" -> "")                  // row marker
>   (1:"v" -> "foo")
> ]
> {noformat}
> Of course, using empty values for the clustering columns doesn't quite work because it
could conflict with the user using empty clustering columns. But in the CompositeType encoding
we have the end-of-component byte that we could reuse by using a specific value (say 0xFF,
currently we never set that byte to anything else than -1, 0 and 1) to indicate it's a static
column.
> With that, we'd need to update the CQL3 statements to support the new syntax and rules,
but that's probably not horribly hard.
> So anyway, this may or may not be a good idea, but I think it has enough meat to warrant
some consideration.



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