Robert Muir created LUCENE-5743:
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Summary: 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
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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