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[Oracle] SQL Execution Engine Part I - Join data by hashing alias Hash Joins

Introduction

We already took a (tiny) closer look at the cost based optimizer in my first series called "DB Optimizer". So i decided to start another tiny series about the SQL execution engine, which executes the calculated and generated execution plan. In the first part of this series i would like to cover a common ABAP coding "mistake", that i notice on regular basis and how it effects the execution / processing by using hash join(s) for two or more tables.

 

The Hash Join

Let's check what hash joins are about before we start to take a look at the execution of hash joins. The following explanation is extracted from the official oracle documentation: