On 08/14/2014 04:01 AM, 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.)
For comparison, here's a patch that implements the scheme that Alexander
Korotkov suggested, where we store an offset every 8th element, and a
length in the others. It compresses Larry's example to 525 bytes.
Increasing the "stride" from 8 to 16 entries, it compresses to 461 bytes.
A nice thing about this patch is that it's on-disk compatible with the
current format, hence initdb is not required.
(The current comments claim that the first element in an array always
has the JENTRY_ISFIRST flags set; that is wrong, there is no such flag.
I removed the flag in commit d9daff0e, but apparently failed to update
the comment and the accompanying JBE_ISFIRST macro. Sorry about that,
will fix. This patch uses the bit that used to be JENTRY_ISFIRST to mark
entries that store a length instead of an end offset.).
- Heikki