static columns and TTL - wouldn't it be nice if static columns played nice with a partition whose partition keys have all (TTL) expired

[graham sanderson]

So, I was thinking about a new use case, where an ideal situation would be to 
have something like

CREATE TABLE series {
        id uuid,
        inserted timeuuid,
        small_thing blob,
        large_static_thing blob static,
        PRIMARY KEY (id, inserted)
}

So this is my first use of static columns, but I also want to use TTL (I just 
built 2.0.8 to play with)

https://issues.apache.org/jira/browse/CASSANDRA-6782 and friends are pretty 
confusing when it comes to TTL and the row marker, but from my playing, it 
seems at least you can control behavior because you can (re) INSERT the primary 
key values only using or not using a TTL. (Side node docs still say UPDATE and 
INSERT are identical which is strictly no longer true)

So what I really want is the ability to do

INSERT INTO series (id, large_static_thing)

then repeated 

INSERT INTO series (id, inserted, small_thing) VALUE (a, b, c) USING TTL X;

and have the partition (and the static column) disappear when the last “row” 
for the partition key is gone.

I can get this behavior if I update the large_static_thing every time along 
with inserting small_thiing, but that is exactly what I don’t want to do 
because it is large and static.

It sort of seems semantically right that "a special column that is shared by 
all the rows of the same partition”  should at least have an option to have it 
expire when all “rows” expire.

It seems like this would be technically feasible (though very much non trivial) 
if you had a syntax, say “large_static_thing blob static autoexpiring”, to make 
the static column an ExpiringColumn, and have any row updates with TTL insert a 
new OnDiskAtom type (that contains a TTL but no value) for the static column. 
These could be reconciled/reduced/compacted or whatever with the ExpiringColumn 
during read and compaction.

It all sounds a bit over-complicated… so:

1) Does this sounds like a useful feature, or is it a me only use case
2) Can someone think of a way to model this reasonably efficiently today 
without using TTL on the static column (and thus having to rewrite it every 
time) - not that I’m trying to be abusive and I haven’t thought this thru, but 
my spider sense makes me think that maybe I can abuse an index on a small 
expiring column to quickly find empty partition keys
3) Is it actually simpler to implement than I think in the code base (This is 
the first time I’ve peeked at these areas of the code)
4) If implemented as I suggested above, does that have to be done in a major 
version?

Thanks,

Graham

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