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Posted to dev@lucene.apache.org by "Michael McCandless (JIRA)" <ji...@apache.org> on 2016/07/12 12:34:20 UTC
[jira] [Commented] (LUCENE-7371) BKDReader could compress values
better
[ https://issues.apache.org/jira/browse/LUCENE-7371?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15372803#comment-15372803 ]
Michael McCandless commented on LUCENE-7371:
--------------------------------------------
This is a nice optimization! Patch looks good!
The {{BKDWriter}} change to pick which dimension to apply the run-length coding to is best effort right? Because, you could have a dim with fewer unique leading suffix bytes, but a larger delta between first and last values? But it would take quite a bit more work at indexing time to figure it out ... maybe add a comment explaining this tradeoff? It seems likely the "min delta" approach should work well in practice, but have you tried with the slow-but-correct approach to verify?
Also, I noticed {{TestBackwardsCompatibility}} seems not to test points! I'll go fix that ...
> BKDReader could compress values better
> --------------------------------------
>
> Key: LUCENE-7371
> URL: https://issues.apache.org/jira/browse/LUCENE-7371
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Adrien Grand
> Assignee: Adrien Grand
> Priority: Minor
> Attachments: LUCENE-7371.patch, LUCENE-7371.patch
>
>
> For compressing values, BKDReader only relies on shared prefixes in a block. We could probably easily do better. For instance there are only 256 possible values for the first byte of the dimension that the values are sorted by, yet we use a block size of 1024. So by using something simple like run-length compression we could save 6 bits per value on average.
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