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Stu Hood commented on CASSANDRA-2319:
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A solution for the key cache is to allow for fuzzy cache entry matches via a
sorted cache structure (if it existed, something like
ConcurrentLinkedSkipListMap would be ideal).
The key cache as it exists gives us exact matches for keys, but when the
resolution of the cache increases to columns, the chance of hitting the same
column twice (while reasonably high) is not high enough. Ideally we'd be able
to fuzzily hit a nearby/next-highest cache entry that represent a range or
block of columns.
An example: the cache contains an entry for columns in the range:
{("user1","entry0100"), ("user1","entry0200")} if a query comes in for a slice
starting from ("user1", "entry0150"), we would perform a fuzzy/floor lookup in
the cache and hit our entry. A lookup that doesn't fall into the range covered
by a cache entry would be a miss, and would result in reading from the index,
and the smallest range bounding the lookup being added to the cache.
> Promote row index
> -----------------
>
> Key: CASSANDRA-2319
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2319
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Stu Hood
> Assignee: Stu Hood
> Labels: index, timeseries
> Fix For: 0.8
>
>
> The row index contains entries for configurably sized blocks of a wide row.
> For a row of appreciable size, the row index ends up directing the third seek
> (1. index, 2. row index, 3. content) to nearby the first column of a scan.
> Since the row index is always used for wide rows, and since it contains
> information that tells us whether or not the 3rd seek is necessary (the
> column range or name we are trying to slice may not exist in a given
> sstable), promoting the row index into the sstable index would allow us to
> drop the maximum number of seeks for wide rows back to 2, and, more
> importantly, would allow sstables to be eliminated using only the index.
> An example usecase that benefits greatly from this change is time series data
> in wide rows, where data is appended to the beginning or end of the row. Our
> existing compaction strategy gets lucky and clusters the oldest data in the
> oldest sstables: for queries to recently appended data, we would be able to
> eliminate wide rows using only the sstable index, rather than needing to seek
> into the data file to determine that it isn't interesting. For narrow rows,
> this change would have no effect, as they will not reach the threshold for
> indexing anyway.
> A first cut design for this change would look very similar to the file format
> design proposed on #674:
> http://wiki.apache.org/cassandra/FileFormatDesignDoc: row keys clustered,
> column names clustered, and offsets clustered and delta encoded.
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