Larry Ellison, Oracle's billionaire co-founder and chairman, said it will be difficult for Amazon to rely exclusively on its own database technologies.

Referring to CNBC's report last week that Amazon is looking to stop using Oracle's technology by early 2020, Ellison said on Tuesday that Amazon wasn't able to get off Oracle years ago. Other competitors, like SAP and Salesforce, have also tried and failed to end their Oracle reliance, Ellison said.

"They've got a goal to get off by 2020," he said at a company event. "SAP couldn't do it. Salesforce couldn't do it. I don't think they can do it. Anyway, we'll find out."

The comments represent the latest in an ongoing spat between the two companies, which compete in the market for cloud infrastructure as well as database systems.

Ellison said Oracle's software is stronger than Amazon's offerings, which include the Aurora database and Redshift data warehouse. He said no database is as easy to use as Oracle's.

They think of themselves as a competitor, so it's kind of embarrassing when Amazon uses Oracle, but they want you to use Aurora and Redshift," he said. "hey've had 10 years to get off Oracle, and they're still on Oracle. And it's not going to be easy for them to use their own technology. It's not going to be cost-effective. I mean, it's really, really hard."

Ellison said he still gives money to Amazon, for other things.

"I just bought the latest Kindle, so it's a great product," he said.

Amazon declined to comment.