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https://issues.apache.org/jira/browse/CASSANDRA-2897?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13279079#comment-13279079
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Jonathan Ellis commented on CASSANDRA-2897:
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bq. it generates both new, correct index entries, AND tombstones for old,
invalid ones (the column entries in the parent CF that get discarded during
compaction
Actually, this doesn't work for us, since (unlike Bigtable) we don't make an
effort to preserve all older versions of a column on disk; we'll happily throw
away old values that got overwritten in the memtable without a trace.
It may be that the best we can do is an alternate index implementation that
does the "purge obsolete entries at read time" but makes no attempt to purge
otherwise (which would make it only suitable for insert-mostly datasets or
read-mostly datasets).
> Secondary indexes without read-before-write
> -------------------------------------------
>
> Key: CASSANDRA-2897
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2897
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Affects Versions: 0.7.0
> Reporter: Sylvain Lebresne
> Priority: Minor
> Labels: secondary_index
>
> Currently, secondary index updates require a read-before-write to maintain
> the index consistency. Keeping the index consistent at all time is not
> necessary however. We could let the (secondary) index get inconsistent on
> writes and repair those on reads. This would be easy because on reads, we
> make sure to request the indexed columns anyway, so we can just skip the row
> that are not needed and repair the index at the same time.
> This does trade work on writes for work on reads. However, read-before-write
> is sufficiently costly that it will likely be a win overall.
> There is (at least) two small technical difficulties here though:
> # If we repair on read, this will be racy with writes, so we'll probably have
> to synchronize there.
> # We probably shouldn't only rely on read to repair and we should also have a
> task to repair the index for things that are rarely read. It's unclear how to
> make that low impact though.
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