Algorithm of Searching 24 Mar 2014

Algorithm

This section is mostly notes on stepping through the Scoring process and serves as fertilizer for the earlier sections.

In the typical search application, a Query is passed to the Searcher , beginning the scoring process.

Once inside the Searcher, a Collector is used for the scoring and sorting of the search results. These important objects are involved in a search:

• The Weight object of the Query. The Weight object is an internal representation of the Query that allows the Query to be reused by the Searcher.
• The Searcher that initiated the call.
• A Filter for limiting the result set. Note, the Filter may be null.
• A Sort object for specifying how to sort the results if the standard score based sort method is not desired.

Assuming we are not sorting (since sorting doesn’t effect the raw Lucene score), we call one of the search methods of the Searcher, passing in the Weight object created by Searcher.createWeight(Query), Filter and the number of results we want. This method returns a TopDocs object, which is an internal collection of search results. The Searcher creates a TopScoreDocCollector and passes it along with the Weight, Filter to another expert search method (for more on the Collector mechanism, see Searcher .) The TopDocCollector uses aPriorityQueue to collect the top results for the search.

If a Filter is being used, some initial setup is done to determine which docs to include. Otherwise, we ask the Weight for a Scorer for the IndexReader of the current searcher and we proceed by calling the score method on the Scorer .

At last, we are actually going to score some documents. The score method takes in the Collector (most likely the TopScoreDocCollector or TopFieldCollector) and does its business. Of course, here is where things get involved. The Scorer that is returned by the Weight object depends on what type of Query was submitted. In most real world applications with multiple query terms, the Scorer is going to be a BooleanScorer2 (see the section on customizing your scoring for info on changing this.)

Assuming a BooleanScorer2 scorer, we first initialize the Coordinator, which is used to apply the coord() factor. We then get a internal Scorer based on the required, optional and prohibited parts of the query. Using this internal Scorer, the BooleanScorer2 then proceeds into a while loop based on the Scorer#next() method. The next() method advances to the next document matching the query. This is an abstract method in the Scorer class and is thus overriden by all derived implementations. If you have a simple OR query your internal Scorer is most likely a DisjunctionSumScorer, which essentially combines the scorers from the sub scorers of the OR’d terms.