# 51.4. The PostgreSQL Rule System

PostgreSQL supports a powerfulrule systemfor the specification ofviewsand ambiguousview updates. Originally the PostgreSQL rule system consisted of two implementations:

  • The first one worked usingrow levelprocessing and was implemented deep in theexecutor. The rule system was called whenever an individual row had been accessed. This implementation was removed in 1995 when the last official release of the Berkeley Postgres project was transformed into Postgres95.

  • The second implementation of the rule system is a technique calledquery rewriting. Therewrite systemis a module that exists between theparser stageand theplanner/optimizer. This technique is still implemented.

    The query rewriter is discussed in some detail inChapter 41, so there is no need to cover it here. We will only point out that both the input and the output of the rewriter are query trees, that is, there is no change in the representation or level of semantic detail in the trees. Rewriting can be thought of as a form of macro expansion.