15.2. Retrieving the Results

After building the Infinispan Query, it can be executed in the same way as a HQL or Criteria query. The same paradigm and object semantic apply to Lucene Query query and all the common operations like list().

15.2.1. Performance Considerations

list() can be used to receive a reasonable number of results (for example when using pagination) and to work on them all. list() works best if the batch-size entity is correctly set up. If list() is used, the Query Module processes all Lucene Hits elements within the pagination.

15.2.2. Result Size

Some use cases require information about the total number of matching documents. Consider the following examples:
Retrieving all matching documents is costly in terms of resources. The Lucene-based Query API retrieves all matching documents regardless of pagination parameters. Since it is costly to retrieve all the matching documents, the Lucene-based Query API can retrieve the total number of matching documents regardless of the pagination parameters. All matching elements are retrieved without triggering any object loads.

Example 15.20. Determining the Result Size of a Query

CacheQuery cacheQuery = Search.getSearchManager(cache).getQuery(luceneQuery,
    Book.class);
//return the number of matching books without loading a single one
assert 3245 == query.getResultSize(); 

CacheQuery cacheQueryLimited =
    Search.getSearchManager(cache).getQuery(luceneQuery, Book.class);
query.maxResults(10);
List results = query.list();
assert 10 == results.size()
//return the total number of matching books regardless of pagination
assert 3245 == query.getResultSize();
The number of results is an approximation if the index is not correctly synchronized with the database. An ansychronous cluster is an example of this scenario.

15.2.3. Understanding Results

Luke can be used to determine why a result appears (or does not appear) in the expected query result. The Query Module also offers the Lucene Explanation object for a given result (in a given query). This is an advanced class. Access the Explanation object as follows:
cacheQuery.explain(int) method

This method requires a document ID as a parameter and returns the Explanation object.

Note

In terms of resources, building an explanation object is as expensive as running the Lucene query. Do not build an explanation object unless it is necessary for the implementation.