Shift (update) unique column values in PostgreSQL

Question

Using MS SQL Server, the following works fine:

CREATE TABLE #temptable(mykey int primary key)

INSERT INTO #temptable VALUES (1)
INSERT INTO #temptable VALUE         


        

Answer 1

Solution without altering constraint as deferrable initially immediate

UPDATE tbl_test t1 
SET    testkey = t2.testkey + 1 
FROM   (SELECT testkey 
    FROM   tbl_test 
    ORDER  BY testkey DESC) t2 
WHERE  t1.testkey = t2.testkey 

Online example: http://rextester.com/edit/GMJ48099