The Query component that returns documents matching a query
The Facet component, for facet.query and facet.field requests where facets are sorted by count (the default). Solr 1.4 and later also support sorting by name.
Each document indexed must have a unique key. (每个doc都要有唯一标识,因为solr要对结果进行合并)
If Solr discovers duplicate document IDs, Solr selects the first document and discards subsequent ones. (solr如果发现重复的id,取首!)
Inverse-document frequency (IDF) calculations cannot be distributed. (idf计算失效,idf牵涉到总文档数,distributed在各个shards进行检索时不方便计算文档总数。)
Distributed searching does not support the QueryElevationComponent, which configures the top results for a given query regardless of Lucene's scoring. For more information, see http://wiki.apache.org/solr/QueryElevationComponent. (QueryElevationComponent不顾及scoring,有用户对结果进行编辑,那么简单的结果合并也就无从谈起。)
The index for distributed searching may become out of date; for example, a document that once matched a query and was subsequently changed may no longer match the query but will still be retrieved. (索引会在distributed searching过程中过时。???)
Distributed searching supports only sorted-field faceting, not date faceting (distributed searching仅支持sorted-field faceting)
The number of shards is limited by number of characters allowed for GET method's URI; most Web servers generally support at least 4000 characters, but many servers limit URI length to reduce their vulnerability to Denial of Service (DoS) attacks. (shards数量受GET地址长度的限制)
TF/IDF computations are per shard. This may not matter if content is well (randomly) distributed. (和第三点类似,tf/idf在各自shard上计算,因此合并出来的scoring排序也不是很“公正”。)