Query with PARALLEL and Without PARALLEL

When you use PARALLEL 4 as a degree of parallelism:
The query will execute using four separate resources (threads or processes) simultaneously. These resources work together to divide and process the query workload in parallel, improving performance for large or complex queries.

When you use a normal SELECT statement without any hint:
The query will execute using a single resource (single thread or process). This is called serial execution, which is sufficient for smaller datasets or less complex operations.

So, the key difference is the number of resources (or execution units) being utilized:

Parallel Query: Uses multiple resources simultaneously.

Serial Query: Uses a single resource.