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https://issues.apache.org/jira/browse/LUCENE-6863?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14982914#comment-14982914
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David Smiley commented on LUCENE-6863:
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bq. For instance, if you look at the cc2 field, about 320k documents have a
value for this field and yet it only takes 1680 bytes of memory for the entire
index, so about 0.005 byte per document. With a hastable, you would either put
data into memory and you could hardly avoid using one or more bytes per
documents, or on disk but then the random-access pattern could be a problem if
not everything fits into your filesystem cache.
If it's sparse (thus small) and the user has chosen to use docValues because
it's going to be sorted/faceted/etc (thus it's likely 'hot' i.e. used) then I
think it's reasonable to expect it'll be in the filesystem cache?
Nonetheless what you've done here is good. I think that an in-memory hash
would be a nice _alternative_ for those that elect for the trade-off. On a
semi-related note, I wonder how well a 4-byte int FST -> long would perform.
bq. Here are the times it take to run these queries on trunk (in milliseconds).
Ah; looks like a fair trade-off to me. Thanks for these details.
> Store sparse doc values more efficiently
> ----------------------------------------
>
> Key: LUCENE-6863
> URL: https://issues.apache.org/jira/browse/LUCENE-6863
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Adrien Grand
> Assignee: Adrien Grand
> Attachments: LUCENE-6863.patch, LUCENE-6863.patch
>
>
> For both NUMERIC fields and ordinals of SORTED fields, we store data in a
> dense way. As a consequence, if you have only 1000 documents out of 1B that
> have a value, and 8 bits are required to store those 1000 numbers, we will
> not require 1KB of storage, but 1GB.
> I suspect this mostly happens in abuse cases, but still it's a pity that we
> explode storage requirements. We could try to detect sparsity and compress
> accordingly.
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