Re: jsonb format is pessimal for toast compression

On 08/20/2014 08:29 AM, Tom Lane wrote:
> Josh Berkus <josh(at)agliodbs(dot)com> writes:
>> On 08/15/2014 04:19 PM, Tom Lane wrote:
>>> Personally I'd prefer to go to the all-lengths approach, but a large
>>> part of that comes from a subjective assessment that the hybrid approach
>>> is too messy. Others might well disagree.
>
>> ... So, that extraction test is about 1% *slower* than the basic Tom Lane
>> lengths-only patch, and still 80% slower than original JSONB. And it's
>> the same size as the lengths-only version.
>
> Since it's looking like this might be the direction we want to go, I took
> the time to flesh out my proof-of-concept patch. The attached version
> takes care of cosmetic issues (like fixing the comments), and includes
> code to avoid O(N^2) penalties in findJsonbValueFromContainer and
> JsonbIteratorNext. I'm not sure whether those changes will help
> noticeably on Josh's test case; for me, they seemed worth making, but
> they do not bring the code back to full speed parity with the all-offsets
> version. But as we've been discussing, it seems likely that those costs
> would be swamped by compression and I/O considerations in most scenarios
> with large documents; and of course for small documents it hardly matters.

Table sizes and extraction times are unchanged from the prior patch
based on my workload.

We should be comparing all-lengths vs length-and-offset maybe using
another workload as well ...

--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com