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https://issues.apache.org/jira/browse/LUCENE-5743?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14021341#comment-14021341
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Adrien Grand commented on LUCENE-5743:
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I'm wondering if we could have another format that would handle the case when
there is a long tail of rare norm values. Eg. if there are 100 unique values
but 95% of documents that have only 3 unique values: we could store norm values
for these 95% documents using TABLE_COMPRESSED (2 bits per value including 1
special value saying that the norm is not there) and the other ones on disk?
> new 4.9 norms format
> --------------------
>
> Key: LUCENE-5743
> URL: https://issues.apache.org/jira/browse/LUCENE-5743
> Project: Lucene - Core
> Issue Type: New Feature
> Reporter: Robert Muir
> Attachments: LUCENE-5743.patch
>
>
> Norms can eat up a lot of RAM, since by default its 8 bits per field per
> document. We rely upon users to omit them to not blow up RAM, but its a
> constant trap.
> Previously in 4.2, I tried to compress these by default, but it was too slow.
> My mistakes were:
> * allowing slow bits per value like bpv=5 that are implemented with expensive
> operations.
> * trying to wedge norms into the generalized docvalues numeric case
> * not handling "simple" degraded cases like "constant norm" the same norm
> value for every document.
> Instead, we can just have a separate norms format that is very careful about
> what it does, since we understand in general the patterns in the data:
> * uses CONSTANT compression (just writes the single value to metadata) when
> all values are the same.
> * only compresses to bitsPerValue = 1,2,4 (this also happens often, for very
> short text fields like person names and other stuff in structured data)
> * otherwise, if you would need 5,6,7,8 bits per value, we just continue to do
> what we do today, encode as byte[]. Maybe we can improve this later, but this
> ensures we don't have a performance impact.
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