How to construct dynamic query over multiple tables - sql

Question

I'm not sure how to formulate this query. I think I need a subquery? Here's basically what I'm trying to do in a single query.
This query gives me the list of tables I need:
SELECT table_name
FROM information_schema.tables
WHERE table_schema = 'abc_dev_12345'
AND table_name like 'fact_%';
For the list of tables given, then I want to do a count from each table_name (each table_name has the same column info I need to query)
SELECT table_name,
count (domain_key) key_count,
domain_key,
form_created_datetime
FROM (List of tables above)
GROUP BY domain_key,
form_created_datetime;
Can I iterate through each table listed in the first query to do my count?
Do this in a single query?
So expected out would be similar to this:
table_name | key_count | domain_key | form_created_datetime
--------------------------------------------------------------
fact_1 1241 5 2015-09-22 01:47:36.136789
fact_2 32 9 2015-09-22 01:47:36.136789
Example data:
abc_dev_12345=> SELECT *
FROM information_schema.tables
where table_schema='abc_dev_own_12345'
and table_name='fact_1';
table_catalog | table_schema | table_name | table_type | self_referencing_column_name | reference_generation | user_defined_type_catalog | user_defined_type_schema | use
r_defined_type_name | is_insertable_into | is_typed | commit_action
---------------+-------------------+--------------------+------------+------------------------------+----------------------+---------------------------+--------------------------+----
--------------------+--------------------+----------+---------------
abc_dev_12345 | abc_dev_own_12345 | fact_1 | BASE TABLE | | | | |
| YES | NO |
(1 row)
abc_dev_12345=> SELECT column_name
FROM information_schema.columns
WHERE table_schema = 'abc_dev_own_12345'
AND table_name = 'fact_1';
column_name
------------------------
email_date_key
email_time_key
customer_key
form_created_datetime
client_key
domain_key

Answer 1

Like Eelke and Craig Ringer noted, you need a dynamic query in a plpgsql function. The basic statement you want to apply to each table is:
SELECT <table_name>, count(domain_key) AS key_count, domain_key, form_created_datetime
FROM <table_name> GROUP BY 3, 4
and you want to UNION the lot together.
The most efficient way to do this is to first build a query as a text object from the information in information_schema.tables and then EXECUTE that query. There are many ways to build that query, but I particularly like the below dirty trick with string_agg():
CREATE FUNCTION table_domains()
RETURNS TABLE (table_name varchar, key_count bigint, domain_key integer, form_created_datetime timestamp)
AS $$
DECLARE
qry text;
BEGIN
-- format() builds query for individual table
-- string_agg() UNIONs queries from all tables into a single statement
SELECT string_agg(
format('SELECT %1$I, count(domain_key), domain_key, form_created_datetime
FROM %1$I GROUP BY 3, 4', table_name),
' UNION ') INTO qry
FROM information_schema.tables
WHERE table_schema = 'abc_dev_12345'
AND table_name LIKE 'fact_%';
-- Now EXECUTE the query
RETURN QUERY EXECUTE qry;
END;
$$ LANGUAGE plpgsql;
No need for loops or cursors so pretty efficient.
Use like you would any other table:
SELECT * FROM table_domains();