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Sylvain Lebresne commented on CASSANDRA-5527:
---------------------------------------------
I agree with Jonathan, I don't see how we could make that efficient without
having to read all the secondary key tombstones each time you read the row,
which doesn't sound fun.
But as an aside, I'll note that another option for this is to use a secondary
index. Now I know it's not read-free, but provided you do provide the partition
key in the query, this will not be horribly inefficient either. And you'll
exchange slightly slower writes for no hit whatsoever on reads, which I would
suspect is a better trade-off more often than not for that kind of operation.
> Deletion by Secondary Key
> -------------------------
>
> Key: CASSANDRA-5527
> URL: https://issues.apache.org/jira/browse/CASSANDRA-5527
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Rick Branson
>
> Given Cassandra's popularity as a time ordered list store, the inability to
> do deletes by anything other than the primary key without re-implementing
> tombstones in the application is a bit of an achilles heel for many use
> cases. It's a data modeling problem that seems to come up quite often, and
> given that we now have the CQL3 abstraction layer sitting on top of the
> storage engine, I think there's an opportunity to take this burden off of the
> application layer. I've spent several weeks thinking about this problem
> within the context of Cassandra, and I think I've come up with a reasonable
> proposal.
> It would involve addition of a secondary key facility to CQL3 tables:
> CREATE TABLE timeline (
> timeline_id uuid,
> entry_id timeuuid,
> entry_key blob,
> entry_payload blob,
> PRIMARY KEY (timeline_id, entry_id),
> KEY (timeline_id, entry_key)
> );
> Secondary keys would be required to share the same partition key with the
> primary key. They would be included to support deletion by secondary key
> operations:
> DELETE FROM timeline WHERE timeline_id = <X> and entry_key = <Y>;
> Underneath, the storage engine row would contain additional secondary key
> tombstones. Secondary key deletion would be read-free, requiring a single
> tombstone write. The cost of reads would necessarily go up. Queries would
> need to be modified to perform an additional step to find any matching
> secondary key tombstones and perform the regular convergence process. The
> secondary key tombstones should be cleaned up by the regular tombstone GC
> process.
> While I didn't want to complicate this idea too much, it might be also worth
> having a discussion around supporting secondary key queries as well, or at
> least making the schema compatible with potential future support (maybe
> rename KEY to DELETABLE KEY or something).
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