Jonathan Lewis’ continuing series on the Oracle optimizer and how it transforms queries into execution plans:
Transformations by the Oracle Optimizer The effects of NULL with NOT IN on Oracle transformations Oracle subquery caching and subquery pushing Oracle optimizer removing or coalescing subqueries Oracle optimizer Or Expansion Transformations
There are two versions of this transformation, the “legacy” Or-Expansion – formerly known as “Concatenation” – and the new (12.2) implementation that allows Oracle greater scope and flexibility in its use. The basic mechanism is essentially the same in both versions though the shape of the execution plan does change.