Re: JSON column disk space

7 Oct 2022

      Hi Ariel,

Let me rephrase the question: Does the process that creates the JSON 
column finish successfully or does the disk get full before it finishes?
The second question is: how many bytes are the keys in each JSON object?

I am trying to understand if the disk usage you see is normal operation 
and what, if anything, can be optimized.

Best regards,
Panos.

On 2022-10-06 19:38, Ariel Abadi wrote:
...
Hi Panos!
I'm not sure I fully understand your question "_If I understand your
issue correctly you don't have the chance to commit the reconstructed
JSON column___"
We are working with auto-commit, if that is the question. but I don't
think the commit would be an issue here.
The problem I saw in the past with json fields and space usage, is
that as you mentioned, JSON fields are being considered as Strings. On
the other hand, as I know (let me know if I'm wrong) MonetDB stores in
the disk the information of every column based on the  DISTINCT values
it may find for every column.
So, if for whatever reason the json keys are saved in different order,
or some keys are not included in some rows, that will become in
different values (based on a DISTINCT values of strings) which will
end up storing in the disk "all the rows" because of the different
values it might encounter in the table.
BR
Ariel
On Thu, Oct 6, 2022 at 1:00 PM Panagiotis Koutsourakis
 wrote:
...
Hello Alberto,
I am looking into this issue and I would like to give you a small
update
and ask a couple of questions.
Some background first. JSON objects are stored in MonetDB as
strings.
This creates overhead from the object keys and from numeric values
that
are now stored as strings. If I understand your issue correctly you
don't have the chance to commit the reconstructed JSON column. Is
this
correct? From some tests I performed, I noticed that even before
committing, disk usage goes up slowly. This is probably unrelated to
JSON itself although I have not had a chance to verify this yet.
So I think the most relevant question at this stage is how long are
the
keys in the JSON objects? I think that these are the most likely
cause
for the large overhead you are observing.
Best regards,
Panos.
...
Niels,
It's hard to me to create a demo script, but at leats I'd want to
know
if it's a known "issue" (maybe it's not an issue, just as Monet
works
internally)
I can tell you that a table with 100 columns (ints, varchars,
dates,
etc) with 30 million records, may occupy 30 Gb of disk.
When I merge some of these columns into one new json column -(i.e.
5
single columns into one object like {col1:value, col2:value...}-
and
dropped the 5 old columns (already merged in json), the table now
grows upto 100 Gb of disk.
Thanks
El jue, 22 sept 2022 a la(s) 10:35, Niels Nes
(niels.nes@monetdbsolutions.com) escribió:
...
Alberto
Do you have some script which replicates this issue?
And could you post the issue on the github issues?
Niels
On Thu, Sep 22, 2022 at 07:57:07AM -0300, Alberto Ferrari wrote:
...
Hi all:
Monet version 11.35.19 (Nov2019-SP3)
We realized that json columns in any table produces bigger
stored
...
...
disk, no matter how many rows has big data.
It seems like depends on rows with different values in same
json
column: if many rows has different values, then the space on
disk
increases exponentially. So we can't use jaon columns in big
On 2022-09-28 21:17, Alberto Ferrari wrote:
tables,
...
...
...
since we run out of space disk in short time.
(also tested with 11.43.5 -Jan2022, same issue)
Is there any solution to this problem?
Tanks!
Alberto
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