使用INSERT ON CONFLICT跨多个列碰撞唯一约束，其中之一可能为null

Question

Evening all,

Say I have a table,

CREATE TABLE test(
col_1 integer,
col_2 integer,
col_3 integer,
CONSTRAINT test_uk UNIQUE (col_1, col_2, col_3));

Insert two rows;

INSERT INTO test (col_1 ,col_2 ,col_3) VALUES (1,2,NULL),(1,2,NULL);

This doesn't raise an error as postgres treats the nulls as not equal. What constraint can I put across these three columns that I can collide on for an update using

INSERT...... ON CONFLICT ... DO UPDATE...;

?

Currently the query will not collide for the above case since, as mentioned, the nulls are not treated as equal.

Thanks in advance.

Answer 1

Not very classy, but you can coalesce the NULL to some "reserved" value, such as 0 or -1. With "reserved" I mean a value that you know it's not going to be part of the potential column values.

create unique index idx_1 on test (coalesce(col_1,-1), coalesce(col_2,-1), coalesce(col_3,-1));