From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Josh Berkus <josh(at)agliodbs(dot)com> |
Cc: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Arthur Silva <arthurprs(at)gmail(dot)com>, Larry White <ljw1001(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Kevin Grittner <kgrittn(at)ymail(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, Bruce Momjian <bruce(at)momjian(dot)us>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Peter Geoghegan <pg(at)heroku(dot)com>, Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz> |
Subject: | Re: jsonb format is pessimal for toast compression |
Date: | 2014-08-26 22:11:33 |
Message-ID: | 11291.1409091093@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
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
Attachment | Content-Type | Size |
---|---|---|
jsonb-lengths-delta.patch | text/x-diff | 13.0 KB |
jsonb-lengths-merged.patch | text/x-diff | 37.7 KB |
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