It is shipping collation-sensitive aggregates between servers which have
different collations.
commit 7012b132d07c2b4ea15b0b3cb1ea9f3278801d98
Author: Robert Haas <rh...@postgresql.org>
Date: Fri Oct 21 09:54:29 2016 -0400
postgres_fdw: Push down aggregates to remote servers.
I've attached a reproducing case. Before this commit, the two final
queries give the same answer, and after they give different answers. Maybe
this isn't considered a bug? Is it just one of the "surprising semantic
anomalies may arise when types or collations do not match"? It should be
able to know what collation the local definition of the foreign table has;
couldn't it pass that collation over the foreign side?
I don't really care, I was just using min as a way to get an arbitrary
value in the cases where there are more than one, but I found the change
surprising.
Cheers,
Jeff
drop database foobar_C;
drop database foobar_US;
create database foobar_C encoding 'utf-8' lc_collate "C" template template0;
create database foobar_US encoding 'utf-8' lc_collate "en_US.utf8" template
template0;
\c foobar_us
create table remote1 (x text);
\copy remote1 from stdin
a
P
\.
\c foobar_c
CREATE EXTENSION IF NOT EXISTS postgres_fdw WITH SCHEMA public;
CREATE SERVER remote_server FOREIGN DATA WRAPPER postgres_fdw options (dbname
'foobar_us') ;
CREATE USER MAPPING FOR postgres SERVER remote_server OPTIONS (
"user" 'postgres'
);
CREATE FOREIGN TABLE remote1 (
x text
)
SERVER remote_server
OPTIONS (
schema_name 'public',
table_name 'remote1',
use_remote_estimate 'true'
);
create table local1 (x text);
\copy local1 from stdin
a
P
\.
select min(x) from local1;
select min(x) from remote1;
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