Re: jsonb format is pessimal for toast compression

On Fri, Aug 8, 2014 at 7:35 PM, Andrew Dunstan <andrew(at)dunslane(dot)net> wrote:
>> I took a quick look and saw that this wouldn't be that easy to get around.
>> As I'd suspected upthread, there are places that do random access into a
>> JEntry array, such as the binary search in findJsonbValueFromContainer().
>> If we have to add up all the preceding lengths to locate the corresponding
>> value part, we lose the performance advantages of binary search. AFAICS
>> that's applied directly to the on-disk representation. I'd thought
>> perhaps there was always a transformation step to build a pointer list,
>> but nope.
>
> It would be interesting to know what the performance hit would be if we
> calculated the offsets/pointers on the fly, especially if we could cache it
> somehow. The main benefit of binary search is in saving on comparisons,
> especially of strings, ISTM, and that could still be available - this would
> just be a bit of extra arithmetic.

I don't think binary search is the main problem here. Objects are
usually reasonably sized, while arrays are more likely to be huge. To
make matters worse, jsonb -> int goes from O(1) to O(n).

Regards,
Ants Aasma
--
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de