Constraints in SQL are faster than python equivalent.
Adding a coalesce in PostgreSQL constraint doesn't seems possible.
This combination of constraint and weird named unique index is the
fastest way to ensure a unique constraint with NULL values.
This index require the website_id to be present in the res_users
table. To do this, a stored readonly related field is used.
This was introduced by 4f6ec1cd2a
Nicolas Seinlet
b5a12b4aac