Type: New Feature
Fix Version/s: 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
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:
then you'd get:
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:
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.