[LUCENE-6172] Improve the in-order / out-of-order collection decision process

• Type: Improvement
• Status: Closed
• Priority: Minor
• Resolution: Won't Fix
• Fix Version/s: 5.0, 6.0
• Lucene Fields: New
• Assignee: Adrien Grand
• Reporter: Adrien Grand
• Created: 09/Jan/15 18:19
• Updated: 28/Aug/22 14:23
• Resolved: 13/Jan/15 15:42

Description

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.

Issue Links

• is superceded by: LUCENE-6179 Remove out-of-order scoring