The answer to your questions is simple: It is much faster to encode from
the original record onto the data pages(s), eliminating the need to
allocate, populate, copy, and release a temporary buffer.
And, frankly, the cost of a byte per full database page is not something to
loose sleep over.
The competitive for a different compression scheme isn't the 30 year old
run length encoding but the self-describing, value driven encoding I
described earlier.
Another area where this is much room for improvement is the encoding of
multi-column indexes. There is a much more clever scheme that doesn't
waste everything fifth byte.
On Friday, February 27, 2015, Slavomir Skopalik <skopa...@elektlabs.cz>
wrote:
> Hi Vlad,
> as I see, in some situation (that really happen), packing into small
> area is padded by zeroes
> (uncomress prefix with zero length).
> And new control char added at begining next fragment (you will lost 2
> bytes).
> The differencies in current compression is not so much, but with better
> one is more significant.
>
> Finally, I still not understand, why is better to compress each fragment
> separatly, instead
> make one compressed block that will split into fragments.
>
> If we have routine to compress/encode full record, we can easyly replace
> curent RLE
> by any other encoding schemna.
>
> In current situation, is not easy replace corent RLE by value encoding
> schema.
>
> I finished new RLE, that is about 25% more efective than my previous post,
> but I lossing lot of bytes on padding and new headers (and also 1 byte
> per row to keep compatibility with previous DB).
>
> I will clean up code and post here durign few days.
>
> Also record differencies encoding can be improoved, I will do if
> somebody will need it.
>
> About update, I'm worry, that fragmented record will not add performace
> gain durign update.
>
> Slavek
>
> > Not exactly so. The big record is prepared for compression as a
> whole, then
> > tail of record is packed and put at separate page(s) and finally what
> left
> > (and could be put on single page) is really "re-compressed" separately.
> >
> >> And when record is materialized in RAM all parts are reads and
> decompress
> >> separatly.
> > What problem do you see here ? How else do you propose to decompress
> fragmented
> > record ?
> >
> >
> >> If comprossor cannot fit in small space, than rest of space is padded
> >> (char 0x0 is in use).
> > Record image in memory always have fixed length, according to record
> format.
> >
> >> This wastage CPU and disk space.
> > CPU - yes, Memory - yes, Disk - no.
> >
> > Also, note, it allows later to not waste CPU when fields are
> accessed and
> > record is updated, AFAIU.
> >
> > Regards,
> > Vlad
> >
> >
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>
>
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> by Intel and developed in partnership with Slashdot Media, is your hub for
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--
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