CQRS simply means "replicate the data", while ES refers to event sourcing. When used together, CQRS/ES entails replicating data on various storage systems by replaying events from the Event Store. This replication can occur continuously on any RDBMS, and you can utilize all of its features, including vendor-specific ones, for querying.

In this setup, your application houses domain-specific code, also known as the "core domain" in DDD terms, while generic code exists in generic tools like RDBMS, called the "generic sub-domain."

For example, report generation is in generic subdomain, and RDBMSes are great at that.