Postgres 16

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select version();

version

PostgreSQL 16.0 (Debian 16.0-1.pgdg120+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit

SELECT 1

create table Marriage(chat_id int, user_id int) ; 

CREATE TABLE

create unique  index chat_user_same_ndx 
    on marriage( least(chat_id, user_id), greatest(chat_id, user_id) );

CREATE INDEX

insert into Marriage (chat_id, user_id) values (1,2);

select * from Marriage;

INSERT 0 1

chat_id
user_id

1
2

SELECT 1

insert into Marriage (chat_id, user_id) values (2,1);

ERROR:  duplicate key value violates unique constraint "chat_user_same_ndx"
DETAIL:  Key (LEAST(chat_id, user_id), GREATEST(chat_id, user_id))=(1, 2) already exists.