| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Bruce Momjian <bruce(at)momjian(dot)us> |
| Cc: | Peter Geoghegan <pg(at)heroku(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, Robert Haas <robertmhaas(at)gmail(dot)com>, Kevin Grittner <kgrittn(at)ymail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Larry White <ljw1001(at)gmail(dot)com> |
| Subject: | Re: jsonb format is pessimal for toast compression |
| Date: | 2014-08-14 01:01:43 |
| Message-ID: | 18845.1407978103@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
I wrote:
> That's a fair question. I did a very very simple hack to replace the item
> offsets with item lengths -- turns out that that mostly requires removing
> some code that changes lengths to offsets ;-). I then loaded up Larry's
> example of a noncompressible JSON value, and compared pg_column_size()
> which is just about the right thing here since it reports datum size after
> compression. Remembering that the textual representation is 12353 bytes:
> json: 382 bytes
> jsonb, using offsets: 12593 bytes
> jsonb, using lengths: 406 bytes
Oh, one more result: if I leave the representation alone, but change
the compression parameters to set first_success_by to INT_MAX, this
value takes up 1397 bytes. So that's better, but still more than a
3X penalty compared to using lengths. (Admittedly, this test value
probably is an outlier compared to normal practice, since it's a hundred
or so repetitions of the same two strings.)
regards, tom lane
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