Swap unique indexed column values in database

The magic word is DEFERRABLE here:

DROP TABLE ztable CASCADE;
CREATE TABLE ztable
    ( id integer NOT NULL PRIMARY KEY
    , payload varchar
    );
INSERT INTO ztable(id,payload) VALUES (1,'one' ), (2,'two' ), (3,'three' );
SELECT * FROM ztable;


    -- This works, because there is no constraint
UPDATE ztable t1
SET payload=t2.payload
FROM ztable t2
WHERE t1.id IN (2,3)
AND t2.id IN (2,3)
AND t1.id <> t2.id
    ;
SELECT * FROM ztable;

ALTER TABLE ztable ADD CONSTRAINT OMG_WTF UNIQUE (payload)
    DEFERRABLE INITIALLY DEFERRED
    ;

    -- This should also work, because the constraint 
    -- is deferred until "commit time"
UPDATE ztable t1
SET payload=t2.payload
FROM ztable t2
WHERE t1.id IN (2,3)
AND t2.id IN (2,3)
AND t1.id <> t2.id
    ;
SELECT * FROM ztable;

RESULT:

DROP TABLE
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "ztable_pkey" for table "ztable"
CREATE TABLE
INSERT 0 3
 id | payload
----+---------
  1 | one
  2 | two
  3 | three
(3 rows)

UPDATE 2
 id | payload
----+---------
  1 | one
  2 | three
  3 | two
(3 rows)

NOTICE:  ALTER TABLE / ADD UNIQUE will create implicit index "omg_wtf" for table "ztable"
ALTER TABLE
UPDATE 2
 id | payload
----+---------
  1 | one
  2 | two
  3 | three
(3 rows)

I think you should go for solution 2. There is no 'swap' function in any SQL variant I know of.

If you need to do this regularly, I suggest solution 1, depending on how other parts of the software are using this data. You can have locking issues if you're not careful.

But in short: there is no other solution than the ones you provided.

Further to Andy Irving's answer

this worked for me (on SQL Server 2005) in a similar situation where I have a composite key and I need to swap a field which is part of the unique constraint.

key: pID, LNUM rec1: 10, 0 rec2: 10, 1 rec3: 10, 2

and I need to swap LNUM so that the result is

key: pID, LNUM rec1: 10, 1 rec2: 10, 2 rec3: 10, 0

the SQL needed:

UPDATE    DOCDATA    
SET       LNUM = CASE LNUM
              WHEN 0 THEN 1
              WHEN 1 THEN 2 
              WHEN 2 THEN 0 
          END
WHERE     (pID = 10) 
  AND     (LNUM IN (0, 1, 2))

There is another approach that works with SQL Server: use a temp table join to it in your UPDATE statement.

The problem is caused by having two rows with the same value at the same time, but if you update both rows at once (to their new, unique values), there is no constraint violation.

Pseudo-code:

-- setup initial data values:
insert into data_table(id, name) values(1, 'A')
insert into data_table(id, name) values(2, 'B')

-- create temp table that matches live table
select top 0 * into #tmp_data_table from data_table

-- insert records to be swapped
insert into #tmp_data_table(id, name) values(1, 'B')
insert into #tmp_data_table(id, name) values(2, 'A')

-- update both rows at once! No index violations!
update data_table set name = #tmp_data_table.name
from data_table join #tmp_data_table on (data_table.id = #tmp_data_table.id)

Thanks to Rich H for this technique. - Mark