jpountz commented on code in PR #935:
URL: https://github.com/apache/lucene/pull/935#discussion_r892164198
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lucene/core/src/java/org/apache/lucene/index/LogMergePolicy.java:
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@@ -568,23 +568,41 @@ public MergeSpecification findMerges(
// Finally, record all merges that are viable at this level:
Review Comment:
> how could it guarantee that the level decided in range of [start,end)
won't contain segments that have lower level than levelBottom?
LogMergePolicy doesn't try to provide this guarantee. It's actually
important it doesn't try to provide this guarantee, otherwise it could end up
with lots of unmerged segments. For instance imagine that you have 9 segments
(S1..S9) on level 10 then one segment (S10) on level 9, one more segment on
level 10 (S11) and then potentially other segments.
If LogMergePolicy refused to merge segments that are on a lower level then
it could never merge together segments S1..S10. This is because segment S10 can
only be merged with segments that are on a higher level because both the
previous and the next segment are on a higher level, and LogMergePolicy only
merges adjacent segments.
This is a downside of LogMergePolicy compared to TieredMergePolicy: because
it only performs merges of adjacent segments, it sometimes has to return merges
where not all segments are on the same level.
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