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https://issues.apache.org/jira/browse/LUCENE-4599?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Adrien Grand updated LUCENE-4599:
---------------------------------
Attachment: LUCENE-4599.patch
New patch (still not committable yet) with better compression ratio thanks to
the following optimizations:
* the block of data compressed by LZ4 only contains term and payload bytes
(without their lengths), everything else (positions, flags, term lengths, etc.)
is stored using packed ints,
* term freqs are encoded in a pfor-like way to save space (this was a 3x/4x
decrease of the space needed to store freqs),
* when all fields have the same flags (a 3-bits int that says whether
positions/offsets/payloads are enabled), the flag is stored only once per
distinct field,
* when both positions and offsets are enabled, I compute average term lengths
and only store the difference between the start offset and the expected start
offset computed from the average term length and the position,
* for lengths, this impl stores the difference between the indexed term length
and the actual length (endOffset - startOffset), with an optimization when they
are always equal to 0 (can happen with ASCII and an analyzer that does not
perform stemming).
Depending on the size of docs, not the same data takes most space in a single
chunk:
|| || Small docs (28 * 1K) || Large doc (1 * 750K) ||
| Total chunk size (positions and offsets enabled) | 21K | 450K |
| Term bytes | 11K (16K before compression) | 64K (84K before compression) |
| Term lengths | 2K | 8K |
| Positions | 3K | 215K |
| Offsets | 3K (4K if positions are disabled) | 150K (240K if positions are
disabled) |
| Term freqs | 500 | 7K |
the rest is negligible
* So with small docs, most of space is occupied by term bytes whereas with
large docs positions and offsets can easily take 80% of the chunk size.
* Compression might not be as good as with stored fields, especially when docs
are large because terms have already been deduplicated.
Overall, the on-disk format is more compact than the Lucene40 term vectors
format (positions and offsets enabled, the number of documents indexed is not
the same for small and large docs):
|| || Small docs || Large docs ||
| Lucene40 tvx | 160033 | 1633 |
| Lucene40 tvd | 49971 | 232 |
| Lucene40 tvf | 11279483 | 56640734 |
| Compressing tvx | 1116 | 78 |
| Compressing tvd | 7589550 | 44633841 |
This impl is 34% smaller than the Lucene40 one on small docs (mainly thanks to
compression) and 21% on large docs (mainly thanks to packed ints). If you have
other ideas to improve this ratio, let me know!
I still have to write more tests, clean up the patch, make reading term vectors
more memory-efficient, and implement efficient merging...
> Compressed term vectors
> -----------------------
>
> Key: LUCENE-4599
> URL: https://issues.apache.org/jira/browse/LUCENE-4599
> Project: Lucene - Core
> Issue Type: Task
> Components: core/codecs, core/termvectors
> Reporter: Adrien Grand
> Assignee: Adrien Grand
> Priority: Minor
> Fix For: 4.1
>
> Attachments: LUCENE-4599.patch, LUCENE-4599.patch
>
>
> We should have codec-compressed term vectors similarly to what we have with
> stored fields.
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