Message-ID: <f875d187-b9a8-4def-896c-a399d05a8723@joeconway.com>
Date: Sun, 3 Dec 2023 12:11:34 -0500
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Subject: Re: Emitting JSON to file using COPY TO
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From: Joe Conway <mail@joeconway.com>
To: Andrew Dunstan <andrew@dunslane.net>, Davin Shearer <davin@apache.org>,
 PostgreSQL-development <pgsql-hackers@postgreSQL.org>
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On 12/3/23 11:03, Joe Conway wrote:
>   From your earlier post, regarding constructing the aggregate -- not
> extensive testing but one data point:
> 8<--------------------------
> test=# copy foo to '/tmp/buf' (format json, force_array);
> COPY 10000000
> Time: 36353.153 ms (00:36.353)
> test=# copy (select json_agg(foo) from foo) to '/tmp/buf';
> COPY 1
> Time: 46835.238 ms (00:46.835)
> 8<--------------------------

Also if the table is large enough, the aggregate method is not even 
feasible whereas the COPY TO method works:
8<--------------------------
test=# select count(*) from foo;
   count
----------
  20000000
(1 row)

test=# copy (select json_agg(foo) from foo) to '/tmp/buf';
ERROR:  out of memory
DETAIL:  Cannot enlarge string buffer containing 1073741822 bytes by 1 
more bytes.

test=# copy foo to '/tmp/buf' (format json, force_array);
COPY 20000000
8<--------------------------

-- 
Joe Conway
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com