When its in PHP, people always proffered LAMP, that is PHP running on Apache server, data stored in MySQL and all this runs on Linux server. Now Rails is slowly bt surely chipping into PHP’s territory. ore people like to start their project with Rails than with PHP (at least the people I know). Or should I say Sinatra as its very easy to learn.
Now there is a change here. MySQL was popular with Rails, but its no more now. Not because its technically inferior to Postgre, but because its owned by Oracle. Oracle is a big company. Its just interested in making profit unlike the free software community which is interested in delivering software to people so that they can benefit out of it. This corporate interest has made MySQL stall. The initial creator of MySQL, Monty Widenus has even started another project called MariaDB, a drop in replacement to MySQL.
But the mood in Rails community is clear. Most Rails apps as far as I know run with PostgreSQL. So learn Postgre. If your app receives lot of hits, possibly tou might dig deep into raw sql to improve its performance. Postgre also has a thing called Geographic Information System in which you can store geographic data, and ask queries like ‘which is the bakery thats nearest to my home?’ and so on.
So if you are interested really and are sure that your app will be popular, use and dig into PostgreSQL.