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From "Robert Muir (JIRA)" <j...@apache.org>
Subject [jira] Created: (LUCENE-2905) Sep codec writes insane amounts of skip data
Date Fri, 04 Feb 2011 17:12:30 GMT
Sep codec writes insane amounts of skip data
--------------------------------------------

                 Key: LUCENE-2905
                 URL: https://issues.apache.org/jira/browse/LUCENE-2905
             Project: Lucene - Java
          Issue Type: Bug
            Reporter: Robert Muir
             Fix For: Bulk Postings branch


Currently, even if we use better compression algorithms via Fixed or Variable Intblock
encodings, we have problems with both performance and index size versus StandardCodec.

Consider the following numbers:

{noformat}
standard:
frq: 1,862,174,204 bytes
prx: 1,146,898,936 bytes
tib: 541,128,354 bytes
complete index: 4,321,032,720 bytes

bulkvint:
doc: 1,297,215,588 bytes
frq: 725,060,776 bytes
pos: 1,163,335,609 bytes
tib: 729,019,637 bytes
complete index: 5,180,088,695 bytes

simple64:
doc: 1,260,869,240 bytes
frq: 234,491,576 bytes
pos: 1,055,024,224 bytes
skp: 473,293,042 bytes
tib: 725,928,817 bytes
complete index: 4,520,488,986 bytes
{noformat}

I think there are several reasons for this:
* Splitting into separate files (e.g. postings into .doc + .freq). 
* Having to store both a relative delta to the block start, and an offset into the block.
* In a lot of cases various numbers involved are larger than they should be: e.g. they are
file pointer deltas, but blocksize is fixed...

Here are some ideas (some are probably stupid) of things we could do to try to fix this:

Is Sep really necessary? Instead should we make an alternative to Sep, Interleaved? that interleaves
doc and freq blocks (doc,freq,doc,freq) into one file? the concrete impl could implement skipBlock()
for when they only want docdeltas: e.g. for Simple64 blocks on disk are fixed size so it could
just skip N bytes. Fixed Int Block codecs like PFOR and BulkVint just read their single numBytes
header they already have today, and skip numBytes.

Isn't our skipInterval too low? Most of our codecs are using block sizes such as 64 or 128,
so a skipInterval of 16 seems a little overkill.

Shouldn't skipInterval not even be a final constant in SegmentWriteState, but instead completely
private to the codec?

For block codecs, doesn't it make sense for them to only support skipping to the start of
a block? Then, their skip pointers dont need to be a combination of delta + upto, because
upto is always zero. What would we have to modify in the bulkpostings api for jump() to work
with this?

For block codecs, shouldn't skipInterval then be some sort of divisor, based on block size
(maybe by default its 1, meaning we can skip to the start of a every block)

For codecs like Simple64 that encode fixed length frames, shouldnt we use 'blockid' instead
of file pointer so that we get smaller numbers? e.g. simple64 can do blockid * 8 to get to
the file pointer.

Going along with the blockid concept, couldnt pointers in the terms dict be blockid deltas
from the index term, instead of fp deltas? This would be smaller numbers and we could compress
this metadata better.


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