Should I query in parallel?

Answer: It depends on system load and server capacity. Even though parallel querying is the default behavior for indexes created in parallel, it usually degrades the overall query throughput on heavily loaded systems.

Parallel queries are optimal for Decision Support System (DSS) and Online Analysis Processing (OLAP). They are also optimal for analytical systems that have large data collections, multiple CPUs with a low number of concurrent users, or Oracle Real Application Clusters (Oracle RAC) nodes.

See Also: “Using Parallel Queries”