Transfer from one DB to next

I'm working with a few people to design a large scale social networking site. We are still looking for our DB guy, so no one on my team can help me with this. My first question is what type of database should we be looking at? I know sql server express limits you to 10gb per database. We don't have much money, so I was thinking open source? Maybe postgres?

Second, if we choose a database, what is involved in transferring to a bigger one? Especially with a live site?

Any thoughts would be great.

Most database systems don't have artificial size limits, and for a website you shouldn't use one that does. MySQL and Postgresql are respectively the #1 and #2 database systems used by PHP programmers. Neither imposes an artificial size limit.

Moving a large application with a large database from one DBMS to a different one would be a truly massive undertaking, on par with the level of effort required to program it originally. It's not something you ever want to have to do.

sorry, i disagree

there is definitely work involved, as the SQL used from one database system to the next is similar but with differences, so you'll be modifying some of your queries

however, it's not a "truly massive undertaking"

Modifying the queries is only a relatively small part of the work that would be involved in migrating a large social network (large enough to have 10GB of database-stored data) from one DBMS to another. Theoretically it shouldn't be too difficult, but I can tell you from personal experience that the code running a large percentage of websites is absolutely hideous.

For a large scale website you have to factor in the time required to re-test the entire site for functionality, performance and, to a lesser extent, security. Since nearly every page on a website depends on the database in some way you can't really get around re-testing the whole site (unless you choose to release untested code).

Then there's the process of actually performing the data migration. I'm not an expert on databases, but I doubt that there is an easy way of seamlessly transferring the data from one system to another without having to shut down the site while the transfer runs. Obviously though it depends on exactly what systems you are migrating between. But, it will likely require more programming to handle the migration, and more testing to ensure that all of the data is migrated properly.

Maybe I exaggerate a bit when I say it would be on par with the level of effort required to program it originally, but I still believe that in most cases it would be a massive undertaking.

Its very true that transferring the DB is not an easy task specially if its need to be done on large scale.Also in case of websites the DB is required to be stored at multiple locations and there may be the problem of scalability which may arise at that point. So, if you really want to transfer the DB into a different format then make sure to retain the backup of DB and after that apply the stored procedures for the same. Even you require an expert DB manager for the same, but you can also do it at your end.