In so many ways, Salesforce.com is the anti-Oracle. Whereas Oracle is determined to sell massive hardware systems loaded with expensive software, Salesforce only offers its business-centric applications over the net. And though Oracle has started offering "cloud" services as well, Salesforce boss Marc Benioff likes to paint his company as a more nimble alternative to the tech offered by his former mentor, Oracle CEO Larry Ellison.

The irony is that Salesforce.com runs on Oracle software – a fact that Oracle CEO Larry Ellison loves to point out while dissing Salesforce and Benioff. But this could change.

Last week, Salesforce.com ran a job listing calling for a number of engineers with experience in PostgreSQL – an open source alternative to databases like those sold by Oracle. Speculation is already swirling that Salesforce is looking for PostgresSQL engineers so that it can ditch Oracle once and for all, and it's easy to see why. The listing – sent to the PostgresSQL jobs mailing list and reported by The New York Times – says that Salesforce is looking to hire five PostgreSQL database engineers this year and between 40 and 50 engineers next year for a "huge project." The engineers are wanted to "design and implement major pieces of the salesforce.com core database infrastructure."

This might not the anti-Oracle project everyone thinks it is. Salesforce spokesperson Andrew Schmitt tells Wired that the listing is actually just for engineers to work for on the hosted PostgreSQL service the company already offers through an application development platform it offers called Heroku, not a migration from Oracle to PostgreSQL. And this summer the company indicated there were no immediate plans to overhaul the software and hardware platform that underpin the company's applications. But as Salesforce continues to grow, a migration from Oracle makes more and more sense.

Back in June, analyst Curt Monash wrote about the possibility that Salesforce.com will move off Oracle's technology and noted that Salesforce has been contributing to Hbase, a NoSQL database based on Google's BigTable and used by Facebook among others.

Why might Salesforce be motivated to ditch Oracle? For one, there's the cost – though Monash speculates that Salesforce has a favorable licensing deal with Oracle. And there's the apparent hostility between Benioff and Ellison.

Benioff spent 13 years working for Oracle and has been described as Ellison's protege. When Benioff left Oracle to found Salesforce in 1999 he not only had Ellison's blessing but an investment as well. But the relationship between the two soured over the years as Ellison shifted his support from Salesforce to Netsuite, a competitor founded by Oracle alum Evan Goldberg.

But last year, when Benioff's speech at Oracle OpenWorld was bumped at the last minute, the relationship went from sour to toxic. Or at least it seemed to. Ellison's continued public digs at Salesforce may make shelling out for Oracle's software too much for Benioff to bear regardless of whether he gets a good deal.

Oracle is still the biggest name in relational databases, and MySQL (which Oracle acquired in 2008) is still the most widely known open source alternative. But interest has been steadily mounting in PostgreSQL thanks to its reputation for high scalability and its ability to handle unstructured data in a way that's usually associated with NoSQL databases. And given Salesforce's commitment to PostgreSQL through Heroku, it could be a good fit for the company.

But one big questions is whether Salesforce customers will go for it. "It is not clear that Postgres could operate at anything like the scale of Salesforce," Quentin Hardy wrote for The New York Times' Bits Blog. "Even if it could, Salesforce would have to spend a lot of time assuring customers that their data would not be affected."

But Ed Boyajian, the CEO of a commercial PostgreSQL vendor called EnterpriseDB, says that PostgreSQL could scale to meet Salesforce's needs.

"Remember, PostgresSQL was born from the same technical whitepaper that Oracle was created from, and it has many of the 'enterprise-grade' characteristics that the large proprietary databases have, such as Multi Version Concurrency Control, point in time recovery and asynchronous replication," he says. "It's highly scalable both in the number of users it can accommodate and in the absolute amount of data it can manage."

Boyajian cites large-scale PostgreSQL deployments at Sony Online Entertainment, which used EnterpriseDB's Postgres Plus Advanced Server product for its massive multi-player online (MMO) games like Free Realms. He also points to Skype, which uses the open source edition of PostgreSQL and has open sourced some of its custom tools.

Other companies that have used PostgreSQL at large scale include Yahoo, used a heavily modified version for its 2-petabyte data warehouse back in 2008.

Even if that job listing concerns the Heroku PostgreSQL service, it shows that Salesforce is gradually moving toward open source software. Acquired by the company in 2010, Heroku is an online app development environment– or "platform cloud" – that has always revolved around open source tools.

At this point, Heroku runs atop Amazon's EC2 service, separate from Salesforce's main applications, which run inside the company's own data centers. But this summer, Salesforce execs Byron Sebastian and Steve Fisher told us that Salesforce planned to offer a version of Heroku in its own data centers at some point in the future. "Customers should have choice when it comes to the infrastructure they're running on," said Sebastian, who has since stepped down as Salesforce's Heroku boss. "We fully expect to give customers additional choices as time goes on."

The trick, Sebastian said, is finding a way that Salesforce and Heroku can run on the same platform. "The Salesforce infrastructure is optimized for the Salesforce cloud. Heroku is different," he said. "There's a non-trivial amount of work required [to move Heroku onto the Salesforce infrastructure]."

He didn't say how this would happen, and judging from our conversation with Fisher, it didn't seem like Salesforce was planning a move off of Oracle. But Monash believes this will just happen behind the scenes. "Some day," he writes, "Marc Benioff will probably say 'We turned off Oracle across most of our applications a while ago, and nobody outside the company even noticed.'"