Re: jsonb format is pessimal for toast compression

I wrote:
> I wish it were cache-friendly too, per the upthread tangent about having
> to fetch keys from all over the place within a large JSON object.

> ... and while I was typing that sentence, lightning struck.  The existing
> arrangement of object subfields with keys and values interleaved is just
> plain dumb.  We should rearrange that as all the keys in order, then all
> the values in the same order.  Then the keys are naturally adjacent in
> memory and object-key searches become much more cache-friendly: you
> probably touch most of the key portion of the object, but none of the
> values portion, until you know exactly what part of the latter to fetch.
> This approach might complicate the lookup logic marginally but I bet not
> very much; and it will be a huge help if we ever want to do smart access
> to EXTERNAL (non-compressed) JSON values.

> I will go prototype that just to see how much code rearrangement is
> required.

This looks pretty good from a coding point of view.  I have not had time
yet to see if it affects the speed of the benchmark cases we've been
trying.  I suspect that it won't make much difference in them.  I think
if we do decide to make an on-disk format change, we should seriously
consider including this change.

The same concept could be applied to offset-based storage of course,
although I rather doubt that we'd make that combination of choices since
it would be giving up on-disk compatibility for benefits that are mostly
in the future.

Attached are two patches: one is a "delta" against the last jsonb-lengths
patch I posted, and the other is a "merged" patch showing the total change
from HEAD, for ease of application.

                        regards, tom lane