Loop through columns of RECORD - postgresql

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.

Answer 2

PL/pgSQL isn't really designed for what you want to do. It doesn't consider a record to be iterable, it's a tuple of possibly different and incompatible data types.
PL/pgSQL has EXECUTE for dynamic SQL, but EXECUTE queries cannot refer to PL/pgSQL variables like NEW or other records directly.
What you can do is convert the record to a hstore key/value structure, then iterate over the hstore. Use each(hstore(the_record)), which produces a rowset of key,value tuples. All values are cast to their text representations.
This toy function demonstrates iteration over a record by creating an anonymous ROW(..) - which will have column names f1, f2, f3 - then converting that to hstore, iterating over its column/value pairs, and returning each pair.
CREATE EXTENSION hstore;
CREATE OR REPLACE FUNCTION hs_demo()
RETURNS TABLE ("key" text, "value" text)
LANGUAGE plpgsql AS
$$
DECLARE
data1 record;
hs_row record;
BEGIN
data1 = ROW(1, 2, 'test');
FOR hs_row IN SELECT kv."key", kv."value" FROM each(hstore(data1)) kv
LOOP
"key" = hs_row."key";
"value" = hs_row."value";
RETURN NEXT;
END LOOP;
END;
$$;
In reality you would never write it this way, since the whole loop can be replaced with a simple RETURN QUERY statement and it does the same thing each(hstore) does anyway - so this is only to show how each(hstore(record)) works, and the above function should never actually be used.

Answer 3

This feature is not supported in plpgsql - Record IS NOT hash array like other scripting languages - it is similar to C or ADA, where this functionality is impossible. You can use other PL language like PLPerl or PLPython or some tricks - you can iterate with HSTORE datatype (extension) or via dynamic SQL
see How to set value of composite variable field using dynamic SQL
But request for this functionality usually means, so you do some wrong. When you use PL/pgSQL you have think different than you use Javascript or Python

Answer 4

FOR data2 IN
SELECT d
from unnest( data1 ) s(d)
LOOP
RETURN NEXT data2;
END LOOP;

Answer 5

If you order your results prior to looping, will you accomplish what you want.
for rc in select * from t1 order by t1.key asc loop
return next rc;
end loop;
will do exactly what you need. It is also the fastest way to perform that kind of task.

Answer 6

I wasn't able to find a proper way to loop over record, so what I did is converted record to json first and looped over json
declare
_src_schema varchar := 'db_utility';
_targetjson json;
_key text;
_value text;
BEGIN
select row_to_json(c.*) from information_schema.columns c where c.table_name = prm_table and c.column_name = prm_column
and c.table_schema = _src_schema into _targetjson;
raise notice '_targetjson %', _targetjson;
FOR _key, _value IN
SELECT * FROM jsonb_each_text(_targetjson)
LOOP
-- do some math operation on its corresponding value
RAISE NOTICE '%: %', _key, _value;
END LOOP;
return true;
end;