Re: jsonb format is pessimal for toast compression

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Peter Geoghegan <pg(at)heroku(dot)com>
Cc: Arthur Silva <arthurprs(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Heikki Linnakangas <hlinnakangas(at)vmware(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>, Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz>
Subject: Re: jsonb format is pessimal for toast compression
Date: 2014-08-27 04:03:42
Message-ID: 18982.1409112222@sss.pgh.pa.us
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Peter Geoghegan <pg(at)heroku(dot)com> writes:
> I'm not surprised that it hasn't beaten HEAD. I haven't studied the
> problem in detail, but I don't think that the "cache awareness" of the
> new revision is necessarily a distinct advantage.

I doubt it's a significant advantage in the current state of the code;
I'm happy if it's not a loss. I was looking ahead to someday fetching key
values efficiently from large EXTERNAL (ie out-of-line-but-not-compressed)
JSON values, analogously to the existing optimization for fetching text
substrings from EXTERNAL text values. As mentioned upthread, the current
JSONB representation would be seriously unfriendly to such a thing.

regards, tom lane

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