Oracle SQL: single SELECT request for multiple owners

Question

I would like your advice on the best method to use.

From a single Oracle server, we have 3 different owners that contain the exact same tables/data structures. Essentially, the owners allow us to separate the data by administrative regions.

Currently, when I want to do a SQL query on the entire data set (regions), I have to do 3 separate queries:

select * from owner1.Table1 union all
select * from owner2.Table1 union all
select * from owner3.Table1

In this simple example, there are no issues, but if the query is complex, it quickly becomes very difficult to maintain.

So, would there be a more efficient way to make only one global query instead of 3. I guess it's possible to do it via a PL/SQL script, or Dynamic SQL, but I don't know...

Basically, I would like to be able to do (where owners would contain the names of my 3 owners):

select * from owners.Table1

It is not possible to build views that would contain the data of all 3 owners (there would be too many).

Thanks

Answer 1

In this simple example, there are no issues, but if the query is complex, it quickly becomes very difficult to maintain.

So, would there be a more efficient way to make only one global query instead of 3.

Use a sub-query factoring clause (aka a CTE) to combine the queries with the simple UNION ALL queries and then perform the complex query on the combined table (rather than trying to perform the queries on the individual tables):

WITH subquery_name AS (
  select * from owner1.Table1 union all
  select * from owner2.Table1 union all
  select * from owner3.Table1
)
SELECT <your complex query>
FROM   subquery_name;