Loop through columns of RECORD

Question

I need to loop through type RECORD items by key/index, like I can do this using array structures in other programming languages.

For example:

DECLARE
    data1    record;
    data2    text;
...
BEGIN
...
FOR data1 IN
    SELECT
        *
    FROM
        sometable
LOOP

    FOR data2 IN
        SELECT
            unnest( data1 )   -- THIS IS DOESN'T WORK!
    LOOP
        RETURN NEXT data1[data2];   -- SMTH LIKE THIS
    END LOOP;

END LOOP;

Answer 1

As @Pavel explained, it is not simply possible to traverse a record, like you could traverse an array. But there are several ways around it - depending on your exact requirements. Ultimately, since you want to return all values in the same column, you need to cast them to the same type - text is the obvious common ground, because there is a text representation for every type.

Quick and dirty

Say, you have a table with an integer, a text and a date column.

CREATE TEMP TABLE tbl(a int, b text, c date);
INSERT INTO tbl VALUES
 (1, '1text',     '2012-10-01')
,(2, '2text',     '2012-10-02')
,(3, ',3,ex,',    '2012-10-03')  -- text with commas
,(4, '",4,"ex,"', '2012-10-04')  -- text with commas and double quotes

Then the solution can be a simple as:

SELECT unnest(string_to_array(trim(t::text, '()'), ','))
FROM   tbl t;

Works for the first two rows, but fails for the special cases of row 3 and 4.
You can easily solve the problem with commas in the text representation:

SELECT unnest(('{' || trim(t::text, '()') || '}')::text[])
FROM   tbl t
WHERE  a < 4;

This would work fine - except for line 4 which has double quotes in the text representation. Those are escaped by doubling them up. But the array constructor would need them escaped by \. Not sure why this incompatibility is there ...

SELECT ('{' || trim(t::text, '()') || '}') FROM tbl t WHERE a = 4

Yields:

{4,""",4,""ex,""",2012-10-04}

But you would need:

SELECT '{4,"\",4,\"ex,\"",2012-10-04}'::text[];  -- works

Proper solution

If you knew the column names beforehand, a clean solution would be simple:

SELECT unnest(ARRAY[a::text,b::text,c::text])
FROM tbl

Since you operate on records of well know type you can just query the system catalog:

SELECT string_agg(a.attname || '::text', ',' ORDER  BY a.attnum)
FROM   pg_catalog.pg_attribute a 
WHERE  a.attrelid = 'tbl'::regclass
AND    a.attnum > 0
AND    a.attisdropped = FALSE

Put this in a function with dynamic SQL:

CREATE OR REPLACE FUNCTION unnest_table(_tbl text)
  RETURNS SETOF text LANGUAGE plpgsql AS
$func$
BEGIN

RETURN QUERY EXECUTE '
SELECT unnest(ARRAY[' || (
    SELECT string_agg(a.attname || '::text', ',' ORDER  BY a.attnum)
    FROM   pg_catalog.pg_attribute a 
    WHERE  a.attrelid = _tbl::regclass
    AND    a.attnum > 0
    AND    a.attisdropped = false
    ) || '])
FROM   ' || _tbl::regclass;

END
$func$;

Call:

SELECT unnest_table('tbl') AS val

Returns:

val
-----
1
1text
2012-10-01
2
2text
2012-10-02
3
,3,ex,
2012-10-03
4
",4,"ex,"
2012-10-04

This works without installing additional modules. Another option is to install the hstore extension and use it like @Craig demonstrates.