Shortcut counts on exists queries

[jpountz]

TopDocsCollectorContext is already able to shortcut hit counts on match_all and term queries when there are no deletions. It would be nice to also shortcut exists queries, especially because running a match_all query on an index that has nested documents runs a DocValueFieldExists query under the hood on the _seq_no field.

This would only work on indices without deletions and fields that are indexed: fields indexed with terms can use Terms#getDocCount and fields indexed with points can use PointValues#getDocCount.

[elasticmachine]

Pinging @elastic/es-search

[cbuescher]

@jpountz working on this, while adding tests to QueryPhaseTests, I discovered the following section in Lucenes IndexSearchers#count method: https://github.com/apache/lucene-solr/blob/master/lucene/core/src/java/org/apache/lucene/search/IndexSearcher.java#L339
This covers the MatchAllDocsQuery and TermQuery in much the same way as TopDocsCollectorContext#shortcutTotalHitCount but could also benefit from adding a section handling the DocValuesFieldExistsQuery, wdyt?

[jpountz]

Unfortunately this optimization is unsafe for Lucene because Lucene doesn't enforce that you are indexing the same data in an indexed and a doc-value field that share the same name. This is something that Elasticsearch enforces however: if a field is indexed and doc-valued, then indexed documents are exactly the same as doc-valued documents.

[cbuescher]

Thanks, thats unfortunate. Do we wrap or overwrite Lucenes IndexSearch in ES in a consistent way somewhere so we could add this optimization only on our side?

[jpountz]

@cbuescher We never use IndexSearcher#count to my knowlegde (maybe we should make it a forbidden API in a separate change?). Counting only occurs in TopDocsCollectorContext#shortcutTotalHitCount.