[
https://issues.apache.org/jira/browse/CASSANDRA-2319?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13008150#comment-13008150
]
Stu Hood commented on CASSANDRA-2319:
-------------------------------------
> I'm not saying anything else. What I'm saying is that there is potentially
> order of magnitudes more 'tuple' than there are keys.
Agreed... my point is simply that the number of columns-per-key and the number
of keys are inversely proportional: if you have more columns-per-key, you have
less keys, and vice-versa. The index will grow proportionally with the total
number of columns, not with the number of keys.
> Also, as tjake remarked, it is unclear how to update the key cache with that
> proposal.
Yea... the key cache as it exists does not necessarily need to change, but at
some point we'll want to update it to include the improvements from this ticket.
> You could cache column position ('tuple') directly, but that will be much
> less useful.
I'm not sure that this would be less useful: promoting the index takes us from
3 to 2 seeks for wide rows: making this particular change to the key cache
would take it from 2 to 1 seek for wide rows (get cached position, seek
directly to column). It depends on the number of unique queries to the row, but
I'm willing to be that the number of unique queries is relatively low.
> 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.
--
This message is automatically generated by JIRA.
For more information on JIRA, see: http://www.atlassian.com/software/jira
