[Feature Request] Concurrent executing query in InnerHitsPhase

[kkewwei]

Is your feature request related to a problem? Please describe

In our product environment, we utilize the nested field and inner_hits, the source contains excessive number of fields, and inner_hits query will match about 80 sub documents within a parent document. we find that the FetchPhase is costly, it will cost 3s+, but when we delete the inner_hits, it will only cost 700ms.

In FetchPhase, each document will execute the InnerHitsPhase serially (regards as sub query phase), if we need to
fetch values from multi documents in FetchPahse, then the overall fetch phase will be very slow.

Describe the solution you'd like

Concurrent executing inner_hits in FetchPhase.

In some case, if the source is too large, it will also cost much time to fetch document in FetchPhase, It also seem necessary to fetch doc concurrently in some scenarios.

Related component

Search:Performance

Describe alternatives you've considered

No response

Additional context

No response

[sandeshkr419]

@jed326 / @sohami - Do you guys have some thoughts here?