On Wed, Aug 13, 2014 at 09:01:43PM -0400, Tom Lane wrote:
> I wrote:
> > That's a fair question. I did a very very simple hack to replace the item
> > offsets with item lengths -- turns out that that mostly requires removing
> > some code that changes lengths to offsets ;-). I then loaded up Larry's
> > example of a noncompressible JSON value, and compared pg_column_size()
> > which is just about the right thing here since it reports datum size after
> > compression. Remembering that the textual representation is 12353 bytes:
>
> > json: 382 bytes
> > jsonb, using offsets: 12593 bytes
> > jsonb, using lengths: 406 bytes
>
> Oh, one more result: if I leave the representation alone, but change
> the compression parameters to set first_success_by to INT_MAX, this
> value takes up 1397 bytes. So that's better, but still more than a
> 3X penalty compared to using lengths. (Admittedly, this test value
> probably is an outlier compared to normal practice, since it's a hundred
> or so repetitions of the same two strings.)
Uh, can we get compression for actual documents, rather than duplicate
strings?
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://enterprisedb.com
+ Everyone has their own god. +
