How does the query optimizer in Oracle work, what does it consider, and to which class does it belong?

The Query optimizer in Oracle is a component of the DBMS responsible for selecting the most efficient execution plan for an SQL query. It analyzes various access paths to data and the order of operations to minimize resource consumption and execution time.

The optimizer belongs to the class of cost-based optimizers (CBO), meaning it evaluates the cost of each possible plan based on data statistics (such as row counts, value distributions, presence of indexes).

Main factors considered by the optimizer:

• Statistics of tables and indexes (row counts, data density)
• Presence and types of indexes
• Filters and conditions in the query
• Relationships between tables (join conditions)
• Available access methods (full scan, index scan, etc.)

Based on this data, the optimizer constructs several plans and selects the one with the lowest estimated cost.

Example: if a table has an index on a column used in the WHERE clause, the optimizer might choose an index scan instead of a full table scan if it estimates this will be faster.