How to select rows that fail exclusion constraint - postgresql

Question

I have a table with rows in it (source) that I am trying to insert into another table (target). The target has an exclusion constraint in place. However, when I do this, some of the rows fail the exclusion constraint. I would like to be able to select these rows in the source, that fail the exclusion constraint. Is this possible?
create table target(
id bigint primary key
,external_data_source_id bigint not null
,external_id text not null
,external_id_domain_id bigint not null
,internal_id bigint not null
,valid_period tstzrange not null
,EXCLUDE USING gist (external_data_source_id with = , external_id_domain_id with =, internal_id with =, external_id with =, valid_period WITH &&)
);
create table source(
id bigint primary key
,external_data_source_id bigint not null
,external_id text not null
,external_id_domain_id bigint not null
,internal_id bigint not null
,valid_period tstzrange not null
);
insert into source
select 1,1,'text',1,1,tstzrange('2000-01-01','2001-01-01');
insert into source
select 2,1,'text',1,1,tstzrange('2000-01-01','2001-01-01');
insert into source
select 1,'text',1,1,tstzrange('2002-01-01','2004-01-01');
insert into target
select * from source;
gives
Error: ERROR: conflicting key value violates exclusion constraint "target_external_data_source_id_external_id_domain_id_inter_excl"
Detail: Key (external_data_source_id, external_id_domain_id, internal_id, external_id, valid_period)=(1, 1, 1, text, ["2000-01-01 00:00:00+01","2001-01-01 00:00:00+01")) conflicts with existing key (external_data_source_id, external_id_domain_id, internal_id, external_id, valid_period)=(1, 1, 1, text, ["2000-01-01 00:00:00+01","2001-01-01 00:00:00+01")).
SQLState: 23P01
ErrorCode: 0
I would like to select the rows in source that fail this exclusion constraint.

Answer 1

You can use condition from the exclusion constraint in an exists query:
select s1.*
from source s1
where exists (select *
from source s2
where (s2.external_data_source_id, s2.external_id_domain_id,
s2.internal_id, s2.external_id)
= (s1.external_data_source_id, s1.external_id_domain_id,
s1.internal_id, s1.external_id)
and s1.valid_period && s2.valid_period
and s1.id <> s2.id
);
Online example: https://rextester.com/PDOE78609