Postgres UPSERT (INSERT or UPDATE) only if value is different

Question

I\'m updating a Postgres 8.4 database (from C# code) and the basic task is simple enough: either UPDATE an existing row or INSERT a new one if one doesn\'t exist yet. Normally I

Answer 1

The RETURNING clause enables you to chain your queries; the second query uses the results from the first. (in this case to avoid re-touching the same rows) (RETURNING is available since postgres 8.4)

Shown here embedded in a a function, but it works for plain SQL, too

DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;

CREATE TABLE my_table
        ( updated_time timestamp NOT NULL DEFAULT now()
        , updated_username varchar DEFAULT '_none_'
        , criteria1 varchar NOT NULL
        , criteria2 varchar NOT NULL
        , value1 varchar
        , value2 varchar
        , PRIMARY KEY (criteria1,criteria2)
        );

INSERT INTO  my_table (criteria1,criteria2,value1,value2)
SELECT 'C1_' || gs::text
        , 'C2_' || gs::text
        , 'V1_' || gs::text
        , 'V2_' || gs::text
FROM generate_series(1,10) gs
        ;

SELECT * FROM my_table ;

CREATE function funky(_criteria1 text,_criteria2 text, _newvalue1 text, _newvalue2 text)
RETURNS VOID
AS $funk$
WITH ins AS (
        INSERT INTO my_table(criteria1, criteria2, value1, value2, updated_username)
        SELECT $1, $2, $3, $4, COALESCE(current_user, 'evgeny' )
        WHERE NOT EXISTS (
                SELECT * FROM my_table nx
                WHERE nx.criteria1 = $1 AND nx.criteria2 = $2
                )
        RETURNING criteria1 AS criteria1, criteria2 AS criteria2
        )
        UPDATE my_table upd
        SET value1 = $3, value2 = $4
        , updated_time = now()
        , updated_username = COALESCE(current_user, 'evgeny')
        WHERE 1=1
        AND criteria1 = $1 AND criteria2 = $2 -- key-condition
        AND (value1 <> $3 OR value2 <> $4 )   -- row must have changed
        AND NOT EXISTS (
                SELECT * FROM ins -- the result from the INSERT
                WHERE ins.criteria1 = upd.criteria1
                AND ins.criteria2 = upd.criteria2
                )
        ;
$funk$ language sql
        ;

SELECT funky('AA', 'BB' , 'CC', 'DD' );            -- INSERT
SELECT funky('C1_3', 'C2_3' , 'V1_3', 'V2_3' );    -- (null) UPDATE 
SELECT funky('C1_7', 'C2_7' , 'V1_7', 'V2_7777' ); -- (real) UPDATE 

SELECT * FROM my_table ;

RESULT:

        updated_time        | updated_username | criteria1 | criteria2 | value1 | value2  
----------------------------+------------------+-----------+-----------+--------+---------
 2013-03-13 16:37:55.405267 | _none_           | C1_1      | C2_1      | V1_1   | V2_1
 2013-03-13 16:37:55.405267 | _none_           | C1_2      | C2_2      | V1_2   | V2_2
 2013-03-13 16:37:55.405267 | _none_           | C1_3      | C2_3      | V1_3   | V2_3
 2013-03-13 16:37:55.405267 | _none_           | C1_4      | C2_4      | V1_4   | V2_4
 2013-03-13 16:37:55.405267 | _none_           | C1_5      | C2_5      | V1_5   | V2_5
 2013-03-13 16:37:55.405267 | _none_           | C1_6      | C2_6      | V1_6   | V2_6
 2013-03-13 16:37:55.405267 | _none_           | C1_8      | C2_8      | V1_8   | V2_8
 2013-03-13 16:37:55.405267 | _none_           | C1_9      | C2_9      | V1_9   | V2_9
 2013-03-13 16:37:55.405267 | _none_           | C1_10     | C2_10     | V1_10  | V2_10
 2013-03-13 16:37:55.463651 | postgres         | AA        | BB        | CC     | DD
 2013-03-13 16:37:55.472783 | postgres         | C1_7      | C2_7      | V1_7   | V2_7777
(11 rows)