Amazon built its business on Oracle databases. They are an Oracle customer no more.:

We migrated 75 petabytes of internal data stored in nearly 7,500 Oracle databases to multiple AWS database services including Amazon DynamoDB, Amazon Aurora, Amazon Relational Database Service (RDS), and Amazon Redshift. The migrations were accomplished with little or no downtime, and covered 100% of our proprietary systems. This includes complex purchasing, catalog management, order fulfillment, accounting, and video streaming workloads. We kept careful track of the costs and the performance, and realized the following results:

• Cost Reduction – We reduced our database costs by over 60% on top of the heavily discounted rate we negotiated based on our scale. Customers regularly report cost savings of 90% by switching from Oracle to AWS.
• Performance Improvements – Latency of our consumer-facing applications was reduced by 40%.
• Administrative Overhead – The switch to managed services reduced database admin overhead by 70%.