First acquaintance of optimizer in ORACLE database

Oracle database is still the most widely used relational database (RDBMS). For the application of relational database, the optimizer is the core part, because it is responsible for parsing SQL, and then accessing the data stored in the database through SQL, that is, the optimizer directly determines the performance of the database. The optimizer (Optimizer) is a core subsystem built into the oracle database. The purpose of the optimizer is to obtain the most efficient execution path (Access Path) of the target SQL in the current situation, that is, to get the optimal execution plan of the target SQL according to certain judgment principles. According to the judgment principle used in choosing and executing the plan, the optimizer of oracle can be divided into two types: RBO (Rule-Based Optimizer, rule-based optimizer) and CBO (cost-based optimizer). Starting with 10g, RBO is no longer supported by oracle.