Tom, here's the results with github data (8 top level keys) only. Here's a
sample object https://gist.github.com/igrigorik/2017462
All-Lenghts + Cache-Aware EXTERNAL
Query 1: 516ms
Query 2: 350ms
The difference is small but I's definitely faster, which makes sense since
cache line misses are probably slightly reduced.
As in the previous runs, I ran the query a dozen times and took the average
after excluding runs with a high deviation.
compare to (copied from my previous email)
HEAD (aka, all offsets) EXTERNAL
Test query 1 runtime: 505ms
Test query 2 runtime: 350ms
All Lengths (Tom Lane patch) EXTERNAL
Test query 1 runtime: 525ms
Test query 2 runtime: 355ms
On Tue, Aug 26, 2014 at 7:11 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> 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