Update only dynamically chosen columns - sql

Question

I've created a function copy_rows_from_table(_tbl regclass) which copies records in a table and gives them a new primary key value. It returns a hstore containing pairs of old_id => new_id. For example for table books my function would create two additional records and return a hstore.
books initially:
id | title | price | author_id | publisher_id
----+---------------+-------+-----------+--------------
1 | The Cyberiad | 15.00 | 23 | 46
2 | The Trial | 10.00 | 12 | 67
books after evaluation of copy_rows_from_table('books'):
id | title | price | author_id | publisher_id
----+---------------+-------+-----------+--------------
1 | The Cyberiad | 15.00 | 23 | 46
2 | The Trial | 10.00 | 12 | 67
3 | The Cyberiad | 15.00 | 23 | 46
4 | The Trial | 10.00 | 12 | 67
returned hstore:
"1"=>"3", "2"=>"4"
It works fine. Now I would like to create a function that copies records from a few tables (passed in an array) and then updates all foreign keys using returned hstore. For example after copying books and authors I want author_id column to be updated in books table. After using my function on books, authors and publishers, if I had a hstore which contains "1"=>"3", "2"=>"4","23"=>"167","12"=>"98","46"=>"87","67"=>"102", my function should update books table in this way:
id | title | price | author_id | publisher_id
----+---------------+-------+-----------+--------------
1 | The Cyberiad | 15.00 | 23 | 46
2 | The Trial | 10.00 | 12 | 67
3 | The Cyberiad | 15.00 | 167 | 87
4 | The Trial | 10.00 | 98 | 102
I came up with something like this:
CREATE OR REPLACE FUNCTION copy_tables(_tbls regclass[])
RETURNS void AS
$func$
DECLARE
_tbl regclass;
_id_pairs hstore;
_table_id_pairs hstore;
_row record;
BEGIN
FOR _tbl IN SELECT _tbls
LOOP
EXECUTE format('SELECT copy_rows_from_table(''%1$s'')', _tbl)
INTO _table_id_pairs;
SELECT COALESCE(_id_pairs, hstore('')) || COALESCE(_table_id_pairs, hstore('')) INTO _id_pairs;
END LOOP;
FOR _tbl IN SELECT _tbls
LOOP
FOR _row IN EXECUTE format('SELECT * FROM %1$s WHERE id = ANY(''%2$s''::uuid[])', _tbl, avals(_id_pairs))
LOOP
EXECUTE (
SELECT format('UPDATE %1$s SET (%2$s) = (%3$s) WHERE id = %4$s'
, _tbl, string_agg(quote_ident(attname), ', '),
string_agg(COALESCE(_id_pairs -> ('_row.' || quote_ident(attname)), '_row.' || quote_ident(attname)), ', '), _row.id)
FROM pg_attribute
WHERE attrelid = _tbl
AND NOT attisdropped
AND attnum > 0
AND attname LIKE '%_id'
);
END LOOP;
END LOOP;
END
$func$
LANGUAGE plpgsql;
But it doesn't quite work. Is there any possibility to update records in a way I explained?

Answer 1

I have finally found a way to do it in PLpgSQL. I just iterate through every column of every record. Here is my working function:
CREATE OR REPLACE FUNCTION copy_tables(_tbls regclass[])
RETURNS void AS
$func$
DECLARE
_id_pairs hstore;
_table_id_pairs hstore;
_row record;
_hs_row record;
BEGIN
FOR I IN array_lower(_tbls, 1)..array_upper(_tbls, 1)
LOOP
EXECUTE format('SELECT copy_rows_from_table(''%1$s'')', _tbls[I])
INTO _table_id_pairs;
SELECT COALESCE(_id_pairs, hstore('')) || COALESCE(_table_id_pairs, hstore('')) INTO _id_pairs;
END LOOP;
FOR I IN array_lower(_tbls, 1)..array_upper(_tbls, 1)
LOOP
FOR _row IN EXECUTE format('SELECT * FROM %1$s WHERE id = ANY(''%2$s''::uuid[])', _tbls[I], avals(_id_pairs))
LOOP
FOR _hs_row IN SELECT kv."key", kv."value" FROM each(hstore(_row)) kv
LOOP
IF _hs_row."value" = ANY(akeys(_id_pairs)) THEN
EXECUTE format('UPDATE %1$s SET %2$s = ''%3$s'' WHERE id = ''%4$s''',
_tbls[I], _hs_row."key", _id_pairs -> _hs_row."value", _row.id);
END IF;
END LOOP;
END LOOP;
END LOOP;
END
$func$
LANGUAGE plpgsql;

Answer 2

PLpgSQL isn't good tool for dynamic updates. Probably there are some few others possibilities, but no one is trivial. The other ways:
Using some more dynamic PL language - PLPythonu, PLPerl
Using extension https://github.com/okbob/pltoolbox - there are functions record_get_fields and record_set_fields