Dynamically add a column with multiple values to any table using a PL/pgSQL function

Question

I would like to use a function/procedure to add to a 'template' table an additional column (e.g. period name) with multiple values, and do a cartesian product on the rows, so my 'template' is duplicated with the different values provided for the new column.

E.g. Add a period column with 2 values to my template_country_channel table:

SELECT * 
FROM   unnest(ARRAY['P1', 'P2']) AS prd(period)
     , template_country_channel 
ORDER BY  period DESC
        , sort_cnty
        , sort_chan;

/* 
-- this is equivalent to: 
(
  SELECT   'P2'::text AS period
         , *
  FROM template_country_channel
) UNION ALL ( 
  SELECT   'P1'::text AS period
         , *
  FROM template_country_channel
)
--
*/

This query is working fine, but I was wondering if I could turn that into a PL/pgSQL function/procedure, providing the new column values to be added, the column to add the extra column to (and optionally specify the order by conditions).

What I would like to do is:

SELECT * 
FROM template_with_periods(
      'template_country_channel'           -- table name
    , ARRAY['P1', 'P2']                    -- values for the new column to be added
    , 'period DESC, sort_cnty, sort_chan'  -- ORDER BY string (optional)
);

and have the same result as the 1st query.

So I created a function like:

CREATE OR REPLACE FUNCTION template_with_periods(template regclass, periods text[], order_by text) 
    RETURNS SETOF RECORD 
AS $BODY$

BEGIN
    RETURN QUERY EXECUTE 'SELECT * FROM unnest($2) AS prd(period), $1 ORDER BY $3' USING template, periods, order_by ;


END;
$BODY$
LANGUAGE 'plpgsql'
;

But when I run:

SELECT * 
FROM template_with_periods('template_country_channel', ARRAY['P1', 'P2'], 'period DESC, sort_cnty, sort_chan');

I have the error ERROR: 42601: a column definition list is required for functions returning “record”

After some googling, it seems that I need to define the list of columns and types to perform the RETURN QUERY (as the error message precisely states). Unfortunately, the whole idea is to use the function with many 'template' tables, so columns name & type lists is not fixed.

Is there any other approach to try?
Or is the only way to make it work is to have within the function, a way to get list of columns' names and types of the template table?

*the whole idea is to use the function with many 'template' tables, so columns name & type lists is not fixed* -- that is not a problem: with `RETURNS SETOF RECORD` you need to enlist the column names and types on the calling side, f.ex. `select * from template_with_periods(...) as t(period text, sort_cnty int, sort_chan text)` — pozs, Apr 20 '17 at 12:53
Actually this would mean i have to specify the columns names/types while writing the query, and I would have to know each table structure -- quite tedious, and this is exactly what I want to avoid (if pgsql allows it). — ant1j, Apr 20 '17 at 16:00
No, unfortunately, it doesn't allow it. You'll need to define the *exact* structure of your output. Either within the function definition, or (with `RETURNS SETOF RECORD`) at the calling side. — pozs, Apr 20 '17 at 16:02

score 0 · Answer 1 · answered Jul 04 '18 at 14:13

I did this with refcursor if You want output columns list completely dynamic:

CREATE OR REPLACE FUNCTION is_record_exists(tablename character varying, columns character varying[], keepcolumns character varying[] DEFAULT NULL::character varying[])
    RETURNS SETOF refcursor AS
$BODY$

DECLARE 
    ref refcursor;
    keepColumnsList text;
    columnsList text; 
    valuesList text;
    existQuery text;
    keepQuery text;
BEGIN
    IF keepcolumns IS NOT NULL AND array_length(keepColumns, 1) > 0 THEN
        keepColumnsList :=  array_to_string(keepColumns, ', ');
    ELSE
        keepColumnsList :=  'COUNT(*)';
    END IF;

    columnsList := (SELECT array_to_string(array_agg(name || ' = ' || value), ' OR ') FROM
        (SELECT unnest(columns[1:1]) AS name, unnest(columns[2:2]) AS value) pair);

    existQuery := 'SELECT ' || keepColumnsList || ' FROM ' || tableName || ' WHERE ' || columnsList;
    RAISE NOTICE 'Exist query: %', existQuery;

    OPEN ref FOR EXECUTE
        existQuery;
    RETURN next ref;
END;$BODY$
  LANGUAGE plpgsql;

Then need to call FETCH ALL IN to get results. Detailed syntax here or there: https://stackoverflow.com/a/12483222/630169. Seems it is the only way for now. Hope something will be changed in PostgreSQL 11 with PROCEDURES.

Dynamically add a column with multiple values to any table using a PL/pgSQL function

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