当列在Postgres中可为空时处理索引冲突

Question

Say I have a table like:

CREATE TABLE test (id SERIAL, first VARCHAR(10), second VARCHAR(10), other VARCHAR(10));
CREATE UNIQUE INDEX unique_index ON test (first, second);

INSERT INTO test (first, second, other)
  VALUES ('lorem', null, 'old');

If I populate this column and have the second field in the index be null, but what to upsert on conflicts where null=null, how can I do this? Currently I'm not getting a conflict when the upsert value is null as well.

INSERT INTO test (first, second, other)
  VALUES ('lorem', null, 'new')
ON CONFLICT (first, second)
  DO UPDATE SET
    other = EXCLUDED.other;

I will get an output of something like this:

1   lorem   (null)  old
2   lorem   (null)  new

This works if I set the second column to any value, but there's no conflict when they're null. Why? And how can I fix this?

Answer 1

You can create an index on an expression. What often works is:

CREATE UNIQUE INDEX unique_index ON test (COALESCE(first, ''), COALESCE(second, ''));

Of course, this assumes that '' is not a valid field value. You can always put something else in . . . such as '<null>' .