fancyerii at gmail
Apr 18, 2012, 8:34 PM
Post #5 of 5
(258 views)
Permalink

Re: why the of advance(int target) function of DocIdSetIterator is defined with uncertain?
[In reply to]


Michael McCandless wrote: So... the good news is I made a new scorer (basically copied DisjunctionMaxScorer and then tweaked from there) that scores the ORonly case. All tests pass w/ this new scorer. And more good news is that if you don't score (I sort by doctitle to do that), you get a speedup – 7.7% in my simplistic test (prefix query unit*, expands to 988 terms, but I force it to do a scoring BQ rewrite, plus force it to use BS2 not BS – the nocommits in the patch). But the bad news is with scoring on it's 22.7% slower! And, the weird news is, I discovered accidentally that BS2 is much (> 2X) faster for this one query. I think we need to modify the criteria that decides whether to use BS or BS2... maybe when there are lots of lowishdocFreq terms, BS2 is better?  why new algorithm is 22% slower than old one? I read the codes and feel them almost the same. On Tue, Apr 17, 2012 at 8:16 PM, Mikhail Khludnev < mkhludnev [at] griddynamics> wrote: > Hello, > > I can't help with the particular question, but can share some experience. > My task is roughly the same I've found the patch > https://issues.apache.org/jira/browse/LUCENE2686 is absolutely useful > (with one small addition, I'll post it in comments soon). By using it I > have disjunction summing query with steady subscorers. > > Regards > > On Tue, Apr 17, 2012 at 2:37 PM, Li Li <fancyerii [at] gmail> wrote: > >> hi all, >> I am now hacking the BooleanScorer2 to let it keep the docID() of the >> leaf scorer(mostly possible TermScorer) the same as the toplevel Scorer. >> Why I want to do this is: When I Collect a doc, I want to know which term >> is matched(especially for BooleanClause whose Occur is SHOULD). we have >> discussed some solutions, such as adding bit masks in disjunction scorers. >> with this method, when we finds a matched doc, we can recursively find >> which leaf scorer is matched. But we think it's not very efficient and not >> convenient to use(this is my proposal but not agreed by others in our >> team). and then we came up with another one: Modifying DisjunctionSumScorer. >> we analysed the codes and found that the only Scorers used by >> BooleanScorer2 that will make the children scorers' docID() not equal to >> parent is an anonymous class inherited from DisjunctionSumScorer. All other >> ones including SingleMatchScorer, countingConjunctionSumScorer(anonymous), >> dualConjuctionSumScorer, ReqOptSumScorer and ReqExclScorer are fit our need. >> The implementation algorithm of DisjunctionSumScorer use a heap to >> find the smallest doc. after finding a matched doc, the currentDoc is the >> matched doc and all the scorers in the heap will call nextDoc() so all of >> the scorers' current docID the nextDoc of currentDoc. if there are N level >> DisjunctionSumScorer, the leaf scorer's current doc is the nth next docId >> of the root of the scorer tree. >> So we modify the DisjuctionSumScorer and let it behavior as we >> expected. And then I wrote some TestCase and it works well. And also I >> wrote some random generated TermScorer and compared the nextDoc(),score() >> and advance(int) method of original DisjunctionSumScorer and modified one. >> nextDoc() and score() and exactly the same. But for advance(int target), we >> found some interesting and strange things. >> at the beginning, I think if target is less than current docID, it >> will just return current docID and do nothing. this assumption let my >> algorithm go wrong. Then I read the codes of TermScorer and found each call >> of advance(int) of TermScorer will call nextDoc() no matter whether current >> docID is larger than target or not. >> So I am confused and then read the javadoc of DocIdSetIterator: >>  javadoc of DocIdSetIterator.advance(int >> target) >> >> int org.apache.lucene.search.DocIdSetIterator.advance(int target) throws >> IOException >> >> Advances to the first beyond (see NOTE below) the current whose document >> number is greater than or equal >> to target. Returns the current document number or NO_MORE_DOCS if there >> are no more docs in the set. >> Behaves as if written: >> int advance(int target) { >> int doc; >> while ((doc = nextDoc()) < target) { >> } >> return doc; >> } >> Some implementations are considerably more efficient than that. >> NOTE: when target < current implementations may opt not to advance beyond >> their current docID(). >> NOTE: this method may be called with NO_MORE_DOCS for efficiency by some >> Scorers. If your >> implementation cannot efficiently determine that it should exhaust, it >> is recommended that you check for >> that value in each call to this method. >> NOTE: after the iterator has exhausted you should not call this method, >> as it may result in unpredicted >> behavior. >>  >> Then I modified my algorithm again and found that >> DisjunctionSumScorer.advance(int target) has some strange behavior. most of >> the cases, it will return currentDoc if target < currentDoc. but in some >> boundary condition, it will not. >> it's not a bug but let me sad. I thought my algorithm has some bug >> because it's advance method is not exactly the same as original >> DisjunctionSumScorer's. >> codes of DisjunctionSumScorer >> @Override >> public int advance(int target) throws IOException { >> if (scorerDocQueue.size() < minimumNrMatchers) { >> return currentDoc = NO_MORE_DOCS; >> } >> if (target <= currentDoc) { >> return currentDoc; >> } >> .... >>  >> for most case if (target <= currentDoc) it will return currentDoc; >> but if previous advance will make sub scorers exhausted, then if may >> return NO_MORE_DOCS >> an example is: >> currentDoc=1 >> minimumNrMatchers=1 >> subScorers: >> TermScorer: docIds: [1, 2, 6] >> TermScorer: docIds: [2, 4] >> after first call advance(5) >> currentDoc=6 >> only first scorer is now in the heap, scorerDocQueue.size()==1 >> then call advance(6) >> because scorerDocQueue.size() < minimumNrMatchers, it just return >> NO_MORE_DOCS >> >> My question is why the advance(int target) method is defined like this? >> for the reason of efficient or any other reasons? >> >> > > > >  > Sincerely yours > Mikhail Khludnev > gedel [at] yandex > > <http://www.griddynamics.com> > <mkhludnev [at] griddynamics> > >
