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From graham sanderson <gra...@vast.com>
Subject Re: static columns and TTL - wouldn't it be nice if static columns played nice with a partition whose partition keys have all (TTL) expired
Date Tue, 24 Jun 2014 05:26:20 GMT
Note, that as I think about it, if you had the new OnDiskAtom time with TTL and no value, then
you wouldn’t need anything special about static columns, you’d just need a CQL syntax
to update/set the TTL for a column which might be useful for lots of things.

On Jun 24, 2014, at 12:22 AM, graham sanderson <graham@vast.com> wrote:

> 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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