Re: transition tables and UPDATE - Mailing list pgsql-hackers

On Wed, 1 Feb 2023 10:03:26 +0100
Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

> Earlier today I gave a talk about MERGE and wanted to provide an example
> with FOR EACH STATEMENT triggers using transition tables.  However, I
> can't find a non-ugly way to obtain the NEW row that corresponds to each
> OLD row ...  I had to resort to an ugly trick with OFFSET n LIMIT 1.
> Can anyone suggest anything better?  I couldn't find any guidance in the
> docs.

What I could come up with is to join old_table and new_table using keys
of the wine table (winery, brand, variety, year), or join them using
values from row_number(), like;

 INSERT INTO wine_audit
  SELECT 'U', now(), row_o, row_n
   FROM (SELECT row_number() OVER() i, row_to_json(o) row_o FROM old_table o) 
   JOIN (SELECT row_number() OVER() i, row_to_json(n) row_n FROM new_table n) 
     USING (i);

> 
> This is the example function I wrote:
> 
> CREATE FUNCTION wine_audit() RETURNS trigger LANGUAGE plpgsql AS $$
>   BEGIN
>     IF (TG_OP = 'DELETE') THEN
>       INSERT INTO wine_audit
>            SELECT 'D', now(), row_to_json(o), NULL FROM old_table o;
>     ELSIF (TG_OP = 'INSERT') THEN
>       INSERT INTO wine_audit
>            SELECT 'I', now(), NULL, row_to_json(n) FROM new_table n;
>     ELSIF (TG_OP = 'UPDATE') THEN
>       DECLARE
>         oldrec record;
>     newrec jsonb;
>     i      integer := 0;
>       BEGIN
>         FOR oldrec IN SELECT * FROM old_table LOOP
>           newrec := row_to_json(n) FROM new_table n OFFSET i LIMIT 1;
>           i := i + 1;
>           INSERT INTO wine_audit
>                SELECT 'U', now(), row_to_json(oldrec), newrec;
>         END LOOP;
>       END;
> 
>     END IF;
>     RETURN NULL;
>   END;
> $$;
> 
> CREATE TABLE wines (winery text, brand text, variety text, year int, bottles int);
> CREATE TABLE shipment (LIKE wines);
> CREATE TABLE wine_audit (op varchar(1), datetime timestamptz,
>              oldrow jsonb, newrow jsonb);
> 
> CREATE TRIGGER wine_update
>   AFTER UPDATE ON wines
>   REFERENCING OLD TABLE AS old_table NEW TABLE AS new_table
>   FOR EACH STATEMENT EXECUTE FUNCTION wine_audit();
> -- I omit triggers on insert and update because the trigger code for those is trivial
> 
> INSERT INTO wines VALUES ('Concha y Toro', 'Sunrise', 'Chardonnay', 2021, 12),
> ('Concha y Toro', 'Sunrise', 'Merlot', 2022, 12);
> 
> INSERT INTO shipment VALUES ('Concha y Toro', 'Sunrise', 'Chardonnay', 2021, 96),
> ('Concha y Toro', 'Sunrise', 'Merlot', 2022, 120),
> ('Concha y Toro', 'Marqués de Casa y Concha', 'Carmenere', 2021, 48),
> ('Concha y Toro', 'Casillero del Diablo', 'Cabernet Sauvignon', 2019, 240);
> 
> ALTER TABLE shipment ADD COLUMN marked timestamp with time zone;
> 
> WITH unmarked_shipment AS
>  (UPDATE shipment SET marked = now() WHERE marked IS NULL
>          RETURNING winery, brand, variety, year, bottles)
> MERGE INTO wines AS w
>      USING (SELECT winery, brand, variety, year,
>                         sum(bottles) as bottles
>                    FROM unmarked_shipment
>                GROUP BY winery, brand, variety, year) AS s
>         ON (w.winery, w.brand, w.variety, w.year) =
>            (s.winery, s.brand, s.variety, s.year)
> WHEN MATCHED THEN
>      UPDATE SET bottles = w.bottles + s.bottles
> WHEN NOT MATCHED THEN
>      INSERT (winery, brand, variety, year, bottles)
>      VALUES (s.winery, s.brand, s.variety, s.year, s.bottles)
> ;
> 
> 
> If you examine table wine_audit after pasting all of the above, you'll
> see this, which is correct:
> 
> ─[ RECORD 1 ]────────────────────────────────────────────────────────────────────────────────────────────────────
> op       │ U
> datetime │ 2023-02-01 01:16:44.704036+01
> oldrow   │ {"year": 2021, "brand": "Sunrise", "winery": "Concha y Toro", "bottles": 12, "variety": "Chardonnay"}
> newrow   │ {"year": 2021, "brand": "Sunrise", "winery": "Concha y Toro", "bottles": 108, "variety": "Chardonnay"}
> ─[ RECORD 2 ]────────────────────────────────────────────────────────────────────────────────────────────────────
> op       │ U
> datetime │ 2023-02-01 01:16:44.704036+01
> oldrow   │ {"year": 2022, "brand": "Sunrise", "winery": "Concha y Toro", "bottles": 12, "variety": "Merlot"}
> newrow   │ {"year": 2022, "brand": "Sunrise", "winery": "Concha y Toro", "bottles": 132, "variety": "Merlot"}
> 
> My question is how to obtain the same rows without the LIMIT/OFFSET line
> in the trigger function.
> 
> 
> Also: how can we "subtract" both JSON blobs so that the 'newrow' only
> contains the members that differ?  I would like to have this:
> 
> ─[ RECORD 1 ]────────────────────────────────────────────────────────────────────────────────────────────────────
> op       │ U
> datetime │ 2023-02-01 01:16:44.704036+01
> oldrow   │ {"year": 2021, "brand": "Sunrise", "winery": "Concha y Toro", "bottles": 12, "variety": "Chardonnay"}
> newrow   │ {"bottles": 108}
> ─[ RECORD 2 ]────────────────────────────────────────────────────────────────────────────────────────────────────
> op       │ U
> datetime │ 2023-02-01 01:16:44.704036+01
> oldrow   │ {"year": 2022, "brand": "Sunrise", "winery": "Concha y Toro", "bottles": 12, "variety": "Merlot"}
> newrow   │ {"bottles": 132}
> 
> -- 
> Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/
> "La gente vulgar sólo piensa en pasar el tiempo;
> el que tiene talento, en aprovecharlo"
> 
> 


-- 
Yugo NAGATA <nagata@sraoss.co.jp>