mikemccand commented on a change in pull request #1948:
URL: https://github.com/apache/lucene-solr/pull/1948#discussion_r505800348
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File path: lucene/core/src/java/org/apache/lucene/index/OrdinalMap.java
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@@ -271,13 +273,26 @@ protected boolean lessThan(TermsEnumIndex a,
TermsEnumIndex b) {
globalOrd++;
}
- this.firstSegments = firstSegments.build();
- this.globalOrdDeltas = globalOrdDeltas.build();
+ long ramBytesUsed = BASE_RAM_BYTES_USED + segmentMap.ramBytesUsed();
+ this.valueCount = globalOrd;
+
+ // If the first segment contains all of the global ords, then we can apply
a small optimization
+ // and hardcode the first segments and global ord deltas as all zeroes.
+ if (ordDeltaBits.length > 0 && ordDeltaBits[0] == 0L &&
ordDeltas[0].size() == this.valueCount) {
Review comment:
> > Do we (somewhere, couldn't find it here) pre-sort all segments by
the cardinality descending?
>
> We do in fact -- the segments are sorted by 'weight', which in all call
sites corresponds to the number of unique terms. This was added in
[LUCENE-5782](https://issues.apache.org/jira/browse/LUCENE-5782).
Ahh, so then we know the first segment will indeed have the most unique
terms, and therefore the highest chance of having "all 0s" ord deltas.
I think 2nd and 3rd segments also might have all 0s ord deltas? But we can
try to optimize that in a followon issue ... progress not perfection!
> > Does our PackedLongValues.monotonicBuilder already optimize for the case
where it is all 0s, for the case where another segment (not the first) has all
the global values as well?
>
> When constructing the individual `PackedInts.Reader` instances, we do
identify the all 0s case and use the lightweight `PackedInts.NullReader`. It's
great we optimize that case, but it does mean this PR doesn't make an enormous
space difference.
Got it. Well, it's great that all these layers optimize :)
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