Dennis Gorelik

ElasticSearch team defends the bloat in ElasticSearch Percolator 5.4
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https://github.com/elastic/elasticsearch/issues/25308
If you're not interested in ranking you can easily turn it off, by wrapping the percolate query in a constant_score query.
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The percolator tries to tag the queries automatically based on the containing query terms. However it can't do this for all percolator queries, because the percolator doesn't know how to extract meaningful information during indexing for all queries. This is a work in progress and will get better over time. It already has shown a significant performance improvement for cases where the percolator was able to analyze the percolator query correctly at index time.
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1) Funny how in order to turn off unneeded feature, application developers have to create an extra wrapper around their query.

2) "work in progress" did not stop ElasticSearch team from breaking backward compatibility and forcing their users to rewrite their legacy code in favor of "work in progress" ElasticSearch 5.4.

3) "a significant performance improvement" is not quantified, and the cases where that improvement happened -
not described.

See also: ElasticSearch Percolator Bloat - part 1