Currently we have a bottleneck for multi-term queries: the dictionary lookup is being done
twice for each term. The first time in Similarity.idf(), where searcher.docFreq() is called.
The second time when the posting list is opened (TermDocs or TermPositions).
The dictionary lookup is not cheap, that's why a significant performance improvement is
possible here if we avoid the second lookup. An easy way to do this is to add a small LRU
cache to TermInfosReader.
I ran some performance experiments with an LRU cache size of 20, and an mid-size index of
500,000 documents from wikipedia. Here are some test results:
50,000 AND queries with 3 terms each:
old: 152 secs
new (with LRU cache): 112 secs (26% faster)
50,000 OR queries with 3 terms each:
old: 175 secs
new (with LRU cache): 133 secs (24% faster)
For bigger indexes this patch will probably have less impact, for smaller once more.
I will attach a patch soon.