PostgreSQL Function returning result set from dynamic tables names - sql

Question

In my database, I have the standard app tables and backup tables. Eg. for a table "employee", I have a table called "bak_employee". The bak_employee table is a backup of the employee table. I use it to restore the employee table between tests.
I'd figure I can use these "bak_" tables to see the changes that have occurred during the test like this:
SELECT * FROM employee EXCEPT SELECT * FROM bak_employee
This will show me the inserted and updated records. I'll ignore the deleted records for now.
Now, what I would like to do is go through all the tables in my database to see if there's any changes in any of the tables. I was thinking of doing this as a function so it's easy to call over and over. This is what I have so far:
CREATE OR REPLACE FUNCTION public.show_diff()
RETURNS SETOF diff_tables AS
$BODY$
DECLARE
app_tables text;
BEGIN
FOR app_tables IN
SELECT table_name
FROM information_schema.tables
WHERE table_catalog = 'myDatabase'
AND table_schema = 'public'
AND table_name not like 'bak_%' -- exclude existing backup tables
LOOP
-- somehow loop through tables to see what's changed something like:
EXECUTE 'SELECT * FROM ' || app_tables || ' EXCEPT SELECT * FROM bak_' || app_tables;
END LOOP;
RETURN;
END;
$BODY$
LANGUAGE plpgsql;
But obviously this isn't going to return me any useful information. Any help would be appreciated.

Answer 1

You cannot return various well-known row types from the same function in the same call. A cheap fix is to cast each row type to text, so we have a common return type.
CREATE OR REPLACE FUNCTION public.show_diff()
RETURNS SETOF text AS -- text!!
$func$
DECLARE
app_table text;
BEGIN
FOR app_table IN
SELECT table_name
FROM information_schema.tables
WHERE table_catalog = 'myDatabase'
AND table_schema = 'public'
AND table_name NOT LIKE 'bak_%' -- exclude existing backup tables
LOOP
RETURN NEXT ' ';
RETURN NEXT '=== ' || app_table || ' ===';
RETURN QUERY EXECUTE format(
'SELECT x::text FROM (TABLE %I EXCEPT ALL TABLE %I) x'
, app_table, 'bak_' || app_table);
END LOOP;
RETURN;
END
$func$ LANGUAGE plpgsql;
Call:
SELECT * FROM public.show_diff();
I had the test suggested by #a_horse at first, but after your comment I realized that there is no need for this. EXCEPT considers NULL values to be equal and shows all differences.
While being at it, I improved and simplified your solution some more. Use EXCEPT ALL: cheaper and does not run the risk of folding complete duplicates.
Using EXCEPT clause in PostgreSQL
TABLE is just syntactical sugar.
Is there a shortcut for SELECT * FROM in psql?
However, if you have an index on a unique (combination of) column(s), a JOIN like I suggested before should be faster: finding the only possible duplicate via index should be substantially cheaper.
Crucial element is the cast the row type to text (x::text).
You can even make the function work for any table - but never more than one at a time: With a polymorphic parameter type:
Refactor a PL/pgSQL function to return the output of various SELECT queries