lucene搜索之facet查询原理和facet查询实例——TODO

转自：http://www.lai18.com/content/7084969.html

Facet说明

我们在浏览网站的时候，经常会遇到按某一类条件查询的情况，这种情况尤以电商网站最多，以天猫商城为例，我们选择某一个品牌，系统会将该品牌对应的商品展示出来，效果图如下：


如上图，我们关注的是品牌，选购热点等方面，对于类似的功能我们用lucene的term查询当然可以，但是在数据量特别大的情况下还用普通查询来实现显然会因为FSDirectory.open等耗时的操作造成查询效率的低下，同时普通查询是全部document都扫描一遍，这样显然造成了查询效率低；

lucene提供了facet查询用于对同一类的document进行聚类化，这样在查询的时候先关注某一个方面，这种显然缩小了查询范围，进而提升了查询效率；

facet模块提供了多个用于处理facet的统计和值处理的方法；

要实现facet的功能，我们需要了解facetField,FacetField定义了dim和此field对应的path,需要特别注意的是我们在做facetField索引的时候，需要事先调用FacetsConfig.build(Document);

FacetField的indexOptions设置为了DOCS_AND_FREQS_AND_POSITIONS的,即既索引又统计出现的频次和出现的位置，这样做主要是为了方便查询和统计；

相应的在存储的时候我们需要利用FacetsConfig和DirectoryTaxonomyWriter；

DirectoryTaxonomyWriter用来利用Directory来存储Taxono信息到硬盘；

DirectoryTaxonomyWriter的构造器如下:

public DirectoryTaxonomyWriter(Directory directory, OpenMode openMode,
      TaxonomyWriterCache cache) throws IOException {

    dir = directory;
    IndexWriterConfig config = createIndexWriterConfig(openMode);
    indexWriter = openIndexWriter(dir, config);

    // verify (to some extent) that merge policy in effect would preserve category docids 
    assert !(indexWriter.getConfig().getMergePolicy() instanceof TieredMergePolicy) : 
      "for preserving category docids, merging none-adjacent segments is not allowed";
    
    // after we opened the writer, and the index is locked, it's safe to check
    // the commit data and read the index epoch
    openMode = config.getOpenMode();
    if (!DirectoryReader.indexExists(directory)) {
      indexEpoch = 1;
    } else {
      String epochStr = null;
      Map<String, String> commitData = readCommitData(directory);
      if (commitData != null) {
        epochStr = commitData.get(INDEX_EPOCH);
      }
      // no commit data, or no epoch in it means an old taxonomy, so set its epoch to 1, for lack
      // of a better value.
      indexEpoch = epochStr == null ? 1 : Long.parseLong(epochStr, 16);
    }
    
    if (openMode == OpenMode.CREATE) {
      ++indexEpoch;
    }
    
    FieldType ft = new FieldType(TextField.TYPE_NOT_STORED);
    ft.setOmitNorms(true);
    parentStreamField = new Field(Consts.FIELD_PAYLOADS, parentStream, ft);
    fullPathField = new StringField(Consts.FULL, "", Field.Store.YES);

    nextID = indexWriter.maxDoc();

    if (cache == null) {
      cache = defaultTaxonomyWriterCache();
    }
    this.cache = cache;

    if (nextID == 0) {
      cacheIsComplete = true;
      // Make sure that the taxonomy always contain the root category
      // with category id 0.
      addCategory(new FacetLabel());
    } else {
      // There are some categories on the disk, which we have not yet
      // read into the cache, and therefore the cache is incomplete.
      // We choose not to read all the categories into the cache now,
      // to avoid terrible performance when a taxonomy index is opened
      // to add just a single category. We will do it later, after we
      // notice a few cache misses.
      cacheIsComplete = false;
    }
}

由上述代码可知，DirectoryTaxonomyWriter先打开一个IndexWriter,在确保indexWriter打开和locked的前提下，读取directory对应的segments中需要提交的内容，如果读取到的内容为空，说明是上次的内容，设置indexEpoch为1，接着对cache进行设置；判断directory中是否还包含有document，如果有设置cacheIsComplete为false,反之为true;