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https://issues.apache.org/jira/browse/LUCENE-2082?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13635788#comment-13635788
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Michael McCandless commented on LUCENE-2082:
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Hi Aleksandra,
I don't think anyone is working on this now ... it'd be quite a bit of work!
The classes have changed names but the core idea is the same. Have a look at
PostingsFormat: that's the Codec component that handles reading/writing/merging
of all postings files (terms dict, docs/freqs/positions/offsets). It seems
like for this issue you'd need to override Fields/Terms/PostingsConsumer.merge
methods.
But some things here will likely require changes outside of Codec, eg today we
always remove deletes while merging, but for this issue it looks like you may
want to have a threshold below which the deletes are not removed...
> Performance improvement for merging posting lists
> -------------------------------------------------
>
> Key: LUCENE-2082
> URL: https://issues.apache.org/jira/browse/LUCENE-2082
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/index
> Reporter: Michael Busch
> Priority: Minor
> Labels: gsoc2013
> Fix For: 4.3
>
>
> A while ago I had an idea about how to improve the merge performance
> for posting lists. This is currently by far the most expensive part of
> segment merging due to all the VInt de-/encoding. Not sure if an idea
> for improving this was already mentioned in the past?
> So the basic idea is it to perform a raw copy of as much posting data
> as possible. The reason why this is difficult is that we have to
> remove deleted documents. But often the fraction of deleted docs in a
> segment is rather low (<10%?), so it's likely that there are quite
> long consecutive sections without any deletions.
> To find these sections we could use the skip lists. Basically at any
> point during the merge we would find the skip entry before the next
> deleted doc. All entries to this point can be copied without
> de-/encoding of the VInts. Then for the section that has deleted docs
> we perform the "normal" way of merging to remove the deletes. Then we
> check again with the skip lists if we can raw copy the next section.
> To make this work there are a few different necessary changes:
> 1) Currently the multilevel skiplist reader/writer can only deal with
> fixed-size
> skips (16 on the lowest level). It would be an easy change to allow
> variable-size skips, but then the MultiLevelSkipListReader can't
> return numSkippedDocs anymore, which SegmentTermDocs needs -> change 2)
> 2) Store the last docID in which a term occurred in the term
> dictionary. This would also be beneficial for other use cases. By
> doing that the SegmentTermDocs#next(), #read() and #skipTo() know when
> the end of the postinglist is reached. Currently they have to track
> the df, which is why after a skip it's important to take the
> numSkippedDocs into account.
> 3) Change the merging algorithm according to my description above. It's
> important to create a new skiplist entry at the beginning of every
> block that is copied in raw mode, because its next skip entry's values
> are deltas from the beginning of the block. Also the very first posting, and
> that one only, needs to be decoded/encoded to make sure that the
> payload length is explicitly written (i.e. must not depend on the
> previous length). Also such a skip entry has to be created at the
> beginning of each source segment's posting list. With change 2) we don't
> have to worry about the positions of the skip entries. And having a few
> extra skip entries in merged segments won't hurt much.
> If a segment has no deletions at all this will avoid any
> decoding/encoding of VInts (best case). I think it will also work
> great for segments with a rather low amount of deletions. We should
> probably then have a threshold: if the number of deletes exceeds this
> threshold we should fall back to old style merging.
> I haven't implemented any of this, so there might be complications I
> haven't thought about. Please let me know if you can think of reasons
> why this wouldn't work or if you think more changes are necessary.
> I will probably not have time to work on this soon, but I wanted to
> open this issue to not forget about it :). Anyone should feel free to
> take this!
> Btw: I think the flex-indexing branch would be a great place to try this
> out as a new codec. This would also be good to figure out what APIs
> are needed to make merging fully flexible as well.
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