Relational Databases: What You Need to Know

Relational databases like Oracle are declarative, set-based engines. You tell them what data you want (using SQL), and the database figures out how to retrieve it. The execution is driven by a component called the optimizer, which chooses the most efficient plan it can—based on statistics, indexes, and data volume.

Every SQL statement is turned into a plan composed of a small number of operations:

• Table access (full scan or index)
• Join (nested loop, hash, merge)
• Filter
• Aggregate
• Sort

The plan is what determines performance.