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From Toke Eskildsen <t...@kb.dk>
Subject Re: Question about threading in search
Date Fri, 17 Aug 2018 09:26:57 GMT
On Sat, 2017-09-02 at 18:33 -0700, Peilin Yang wrote:
> we're comparing two different indexes on the same collection - one
> with lots of different segments (default settings), and one with a
> force merged into one segment. It seems that search is sometimes
> faster with multiple segments.

If you are using Lucene 7+ and if some of the fields you are requesting
as part of your search result are stored as DocValues, you might have
encountered a performance regression with the streaming API:
https://issues.apache.org/jira/browse/LUCENE-8374

One peculiar effect of this issue is that fewer larger segments gets
slower DocValues retrieval, compared to more smaller segments. So a
force merge to 1 segment can result in worse performance.

- Toke Eskildsen, the Royal Danish Library, Denmark


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