Comparing the data in two Postgres databases I am looking for a way to compare the data in two Postgres databases.

Basically a 'Before' and 'After' snapshot of a transaction being posted.  What I am looking for is all the tables/records that have been altered by a particular type of transaction.

The solution could use ODBC, or PostgreSQL directly. I have connections to the databases both ways.
This question (in some shape or form - no two questions are identical), for example see the threads here and here - I did answer there.

The two most popular (Open Source) tools in this area are Liquibase and Flyaway. There are numerous commercial tools which you can find by Googling.

I would like to echo Paul White's welcome, but just a wee word of warning. Tool recommendations aren't really on-topic here. Check out the tour and also the "help us to help you" blog - both at the bottom left of the page. Also, take a look at the help centre for everything you could ever want to know about dba.se. These forums are a great resource and you'll get the most from them if you take the time to check out and follow the guidelines.

Googling the term "diff tool for postgresql" gives some other ideas. Perhaps a good approach would be to try out a couple (few) of them and if you're having problems or require more specific advice (too broad is also off-topic :-( ) - I got this and this - PostgresQL specific. Best of luck.