Today the logic is the following:
- IndexSearcher looks if the weight can score out-of-order
- Depending on the value it creates the appropriate top docs/field collector
I think this has several issues:
- Only IndexSearcher can actually make the decision correctly, and it only works for top docs/field collectors. If you want to make a multi collector in order to have both facets and top docs, then you're clueless about whether you should create a top docs collector that supports out-of-order collection
- It is quite fragile: you need to make sure that Weight.scoresDocsOutOfOrder and Weight.bulkScorer agree on when they can score out-of-order. Some queries like BooleanQuery duplicate the logic and other queries like FilteredQuery just always return true to avoid complexity. This is inefficient as this means that IndexSearcher will create a collector that supports out-of-order collection while the common case actually scores documents in order (leap frog between the query and the filter).
Instead I would like to take advantage of the new collection API to make out-of-order scoring an implementation detail of the bulk scorers. My current idea is as follows:
- remove Weight.scoresDocsOutOfOrder
- change Collector.getLeafCollector(LeafReaderContext) to Collector.getLeafCollector(LeafReaderContext, boolean canScoreOutOfOrder)
This new boolean in Collector.getLeafCollector tells the collector that the scorer supports out-of-order scoring. So by returning a leaf collector that supports out-of-order collection, things will be faster.
The new logic would be the following. First Weights cannot tell whether they support out-of-order scoring or not. However when a weight knows it supports out-of-order scoring, it will pass canScoreOutOfOrder=true when getting the leaf collector. If the returned collector accepts documents out of order, then the weight will return an out-of order scorer. Otherwise, an in-order scorer is returned.