Doing the computation near the data is almost always a performance win, as the latencies to get the data are minimal. In a typical data-intensive computation, most of the time is spent in getting the data. Therefore, making data access inside the computation faster is the best way to make the whole thing fast. On my laptop, it takes 2.2 ms to query one random row from a 1,000,000-row database into the client, but it takes only 0.12 ms to get the data inside the database. This is 20 times faster and inside the same machine over Unix sockets. The difference can be bigger if there is a network connection between the client and the server.

A small real-word story:

A friend of mine was called to help a large company (I'm sure all of you know it, but I can't tell you which one) in order to make its e-mail sending application faster. They had implemented their e-mail generation system with all the latest Java EE technologies: first, getting the data from the database, passing the data around between services, and serializing and deserializing it several times before finally doing XSLT transformation on the data to produce the e-mail text. The end result being that it produced only a few hundred e-mails per second, and they were falling behind with their responses.

When he rewrote the process to use a PL/Perl function inside the database to format the data and the query returned already fully-formatted e-mails, it suddenly started spewing out tens of thousands of e-mails per second and they had to add a second copy of the sent mail to actually be able to send them out.

If all the data manipulation code is in a database, either as database functions or views, the actual upgrade process becomes very easy. All that is needed is to run a DDL script that redefines the functions; all the clients automatically use the new code with no downtime and no complicated coordination between several frontend systems and teams.

Server-side functions are perhaps the best way to achieve code reuse. Any client application written in any language or framework can make use of the server-side functions, ensuring maximum reuse in all environments.

If all the access for some possibly insecure servers goes through functions, the database user of these servers can only be granted access to the needed functions and nothing else. They can't see the table data or even the fact that these tables exist. So, even if the server is compromised, all it can do is continue to call the same functions. Also, there is no possibility of stealing passwords, e-mails, or other sensitive information by issuing its own queries such as SELECT * FROM users; and getting all the data there is in the database.

Also, the most important thing is that programming in a server is fun!