How to add a constraint to a table using a partial index? I have a simple table:

create table item (
  lan      text not null,
  disabled date
);


My aim is to ensure uniqueness for the lan column over all rows, where disabled is null.

I created a partial index:

create unique index lan_idx on item (lan) where disabled is not null;


Now I tried to add the index to the table, but I am struggling with the syntax. (I took the code from this tutorial)

alter table item add constraint lan_idx using index lan_idx;


This throws an syntax error before "using".

I also tried the following:

alter table item add constraint lan_idx unique using index lan_idx;


But this throws the error, that a unique constraint can not be created with a partial index.

Can anybody tell me what I did wrong?
You can't define a unique (or primary key) constraint based on a partial index.

However, to enforce uniqueness of the lan column for all non-deleted rows, the partial unique index you already have is enough. 

There is no need to add a constraint.