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From "Stu Hood (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-2319) Promote row index
Date Fri, 15 Apr 2011 05:17:05 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2319?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13020192#comment-13020192
] 

Stu Hood commented on CASSANDRA-2319:
-------------------------------------

The attached patch depends on 2336 and 2398, and implements a compressed, promoted index (called
NestedIndex) containing enough information to eliminate wide rows without seeking into the
data file. Only some of the features from the issue description are actually implemented:
the primary goal of this first version was to begin eliminating sstables for wide-row (time
series) usecases.

The nested index contains nearly as much information as our current row header: the only thing
missing at this point is the bloom filter (which I think needs more thought to get right,
and wasn't critical for our slice usecase).

Narrow rows (rows < column_index_size) are represented in the index by their key, offset
and two bits to indicate that neither metadata nor column names have been stored in the index.
Rows wider than column_index_size will have an entry containing their metadata and one or
more column entries, with a bit per column entry to indicate their ownership.

The key cache stores RowHeaders, which have a base implementation that is essentially just
a boxed long offset (matching our existing impl in memory usage). Wide rows use the NestedRowHeader
subclass, which additionally contains row metadata and the min and max columns in the row,
which is enough to eliminate a row for slices. The intention is that as our file format migrates
toward being block based, RowHeader will evolve into a BlockHeader describing a seekable point
in the data file (and possibly cached in a sorted structure as described above).

Data in the index mostly uses the design from [[FileFormatDesignDoc|http://wiki.apache.org/cassandra/FileFormatDesignDoc]],
and is compressed using either type specific compression or LZF (via CASSANDRA-2398). The
end result is that for a narrow-row usecase and simple keys, the index is half the size of
the existing implementation: complex keys will likely see larger benefits.

> 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: 1.0
>
>         Attachments: 2319-v1.tgz, promotion.pdf, version-f.txt, version-g-lzf.txt, version-g.txt
>
>
> 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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