Question

I’d like to add a unique key to column value but I must ignore rows that have the same values in columns value and header_id. For example, consider this table:

id | header_id | value
 1 |         1 | a
 2 |         1 | a
 3 |         2 | a

So rows 1 and 2 point to same object and the unique key should accept them, but row 3 has a different header_id (pointing to another object) and, because it has the same value as object 1, it should violate unique constraint and raise an error.

Edit 16.2:1327:
I’m using a core framework that generates columns to handle history so I cannot normalize the table. My class has lots of columns but for this example I’m only considering the value column.

Answer 1

After a while I found something. Using constrain CHECK with function to determine if exist (Cannot use SELECT in CHECK statement but you can use function with desired select)

CREATE OR REPLACE FUNCTION is_value_free(_header_id integer, _value varchar) RETURNS BOOLEAN AS 
$$
  BEGIN
    RETURN NOT EXISTS (SELECT header_id,value FROM myschema.mytalbe WHERE value LIKE _value AND header_id != _header_id LIMIT 1);
  END;
$$ LANGUAGE plpgsql;

ALTER TABLE mytable ADD CONSTRAINT uniq_value CHECK (is_value_free(header_id,value))