Filter context no longer used

[JacobBrandt]

Kibana version: Since at least 5.3

Filter context no longer gets used like it used to when creating filters. You can no longer affect the score of returned documents by placing clauses in a filter instead of defining that clause in a query. You use to be able to have filter in the filter bar that would put that part of the request in the filter context. Now every non negated filter is going into the must section which is part of the query context and not the filter context.

This is worse behavior because frequently used filters will be cached automatically by Elasticsearch to speed up performance. But since a lot of clauses are not getting put in the filter context we're losing this performance benefit. Our users complained when we moved to 5.3 but we thought it was more of a cluster issue. Now we know that Kibana stopped using filter context performance benefits for filters.

[elasticmachine]

Pinging @elastic/kibana-app

[Bargs]

Duplicate of #8367