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From "Michael Busch (JIRA)" <j...@apache.org>
Subject [jira] Created: (LUCENE-687) Performance improvement: Lazy skipping on proximity file
Date Wed, 18 Oct 2006 09:17:34 GMT
Performance improvement: Lazy skipping on proximity file
--------------------------------------------------------

                 Key: LUCENE-687
                 URL: http://issues.apache.org/jira/browse/LUCENE-687
             Project: Lucene - Java
          Issue Type: Improvement
          Components: Index
            Reporter: Michael Busch
            Priority: Minor


Hello,

I'm proposing a patch here that changes org.apache.lucene.index.SegmentTermPositions to avoid
unnecessary skips and reads on the proximity stream. Currently a call of next() or seek(),
which causes a movement to a document in the freq file also moves the prox pointer to the
posting list of that document.  But this is only necessary if actual positions have to be
retrieved for that particular document. 

Consider for example a phrase query with two terms: the freq pointer for term 1 has to move
to document x to answer the question if the term occurs in that document. But *only* if term
2 also matches document x, the positions have to be read to figure out if term 1 and term
2 appear next to each other in document x and thus satisfy the query. 

A move to the posting list of a document can be quite expensive. It has to be skipped to the
last skip point before that document and then the documents between the skip point and the
desired document have to be scanned, which means that the VInts of all positions of those
documents have to be read and decoded. 

An improvement is to move the prox pointer lazily to a document only if nextPosition() is
called. This will become even more important in the future when the size of the proximity
file increases (e. g. by adding payloads to the posting lists).

My patch implements this lazy skipping. All unit tests pass. 


I also attach a new unit test that works as follows:
Using a RamDirectory an index is created and test docs are added. Then the index is optimized
to make sure it only has a single segment. This is important, because IndexReader.open() returns
an instance of SegmentReader if there is only one segment in the index. The proxStream instance
of SegmentReader is package protected, so it is possible to set proxStream to a different
object. I am using a class called SeeksCountingStream that extends IndexInput in a way that
it is able to count the number of invocations of seek(). 

Then the testcase searches the index using a PhraseQuery "term1 term2". It is known how many
documents match that query and the testcase can verify that seek() on the proxStream is not
called more often than number of search hits.

Example:
Number of docs in the index: 500
Number of docs that match the query "term1 term2": 5

Invocations of seek on prox stream (old code): 29
Invocations of seek on prox stream (patched version): 5

- Michael


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