Deferrable unique index in postgres

A index cannot be deferred - doesn't matter if it is UNIQUE or not, partial or not, only a UNIQUE constraint. Other types of constraints (FOREIGN KEY, PRIMARY KEY, EXCLUDE) are also deferrable - but not CHECK constraints.

So the unique partial index (and the implicit constraint it implements) will be checked at every statement (and in fact after every row insert/update in current implementation), not at the end of transaction.

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What you could do, if you want to implement this constraint as deferrable, is to add one more table in the design. Something like this:

CREATE TABLE public.booking_status
  ( booking_id int NOT NULL,               -- same types
    check_in timestamp NOT NULL,           -- as in  
    check_out timestamp NOT NULL,          -- booking
    CONSTRAINT unique_booking
        UNIQUE (check_in, check_out)
        DEFERRABLE INITIALLY DEFERRED,
    CONSTRAINT unique_booking_fk
        FOREIGN KEY (booking_id, check_in, check_out)
        REFERENCES public.booking (booking_id, check_in, check_out)
        DEFERRABLE INITIALLY DEFERRED
  ) ;

With this design and assuming that booking_status has only 2 possible options (0 and 1), you could remove it entirely from booking (if there is a row at booking_status, it's 1, if not is 0).

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Another way would be to (ab)use an EXCLUDE constraint:

ALTER TABLE booking
    ADD CONSTRAINT unique_booking
        EXCLUDE 
          ( check_in  WITH =, 
            check_out WITH =, 
            (CASE WHEN booking_status = 1 THEN TRUE END) WITH =
          ) 
        DEFERRABLE INITIALLY DEFERRED ;

Tested at dbfiddle.

What the above does:

• The CASE expression becomes NULL when booking_status is null or different than 1. We could write (CASE WHEN booking_status = 1 THEN TRUE END) as (booking_status = 1 OR NULL) if that makes it any more clear.

• Unique and exclude constraints accept rows where one or more of the expressions is NULL. So it acts as a filtered index with WHERE booking_status = 1.

• All the WITH operators are = so it acts as a UNIQUE constraint.

• These two combined make the constraint act as a filtered unique index.

• But it's a constraint and EXCLUDE constraints can be deferred.