What exactly are your techniques of connecting data spread over multiple databases architectures (think MySQL versus PostgreSQL etc), right into a single application?

Can you create giant hashtables/arrays to complement content against each other? Exist other, more efficient and fewer memory-consuming choices for carrying this out?

Should you use data both from the MySQL &lifier PostgreSQL source, without any method of transforming one DB towards the other (application constraints, insufficient time, insufficient understanding, ... ), how does one do it?

SQL Relay or any other sql proxy. http://sqlrelay.sourceforge.net/

A minimum of within the situation of MySQL, you should use data from multiple databases in one query anyway, provided the databases are located through the same MySQL Server instance. You are able to distinguish tables from different databases by being approved the table having a schema title:

CREATE TABLE test.foo (id SERIAL PRIMARY KEY) TYPE=InnoDB;

CREATE DATABASE test2;
CREATE TABLE test2.bar (foo_id BIGINT UNSIGNED, 
    FOREIGN KEY (foo_id) REFERENCES test.foo(id)) TYPE=InnoDB;

SELECT * FROM test.foo f JOIN test2.bar b ON (f.id = b.foo_id);

In PostgreSQL, you may also qualify table references having a schema title. I am unsure if you're able to create foreign key constraints across databases, though.

If you are searching to produce constraints across RDBMSes - you cannot.

I am facing exactly the same problem with running a part of a credit card applicatoin off PostgreSQL for where it'll benefit, and also the relaxation of MySQL where it's better.

I am doing multiple card inserts keyed from the same format of primary information (during my situation a normal user ID), so I am letting the applying handle the logic of creating certain to request for the similar ID from both DBs.

There's not just a clean method of doing this outdoors of abstracting it to some class or utility function, though, that I have found.