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><A
NAME="RULE-SYSTEM"
>44.4. The <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> Rule System</A
></H1
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports a powerful
<I
CLASS="FIRSTTERM"
>rule system</I
> for the specification
of <I
CLASS="FIRSTTERM"
>views</I
> and ambiguous <I
CLASS="FIRSTTERM"
>view updates</I
>.
Originally the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
rule system consisted of two implementations:
<P
></P
></P><UL
><LI
><P
> The first one worked using <I
CLASS="FIRSTTERM"
>row level</I
> processing and was
implemented deep in the <I
CLASS="FIRSTTERM"
>executor</I
>. 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 <SPAN
CLASS="PRODUCTNAME"
>Berkeley Postgres</SPAN
> project was
transformed into <SPAN
CLASS="PRODUCTNAME"
>Postgres95</SPAN
>.
</P
></LI
><LI
><P
> The second implementation of the rule system is a technique
called <I
CLASS="FIRSTTERM"
>query rewriting</I
>.
The <I
CLASS="FIRSTTERM"
>rewrite system</I
> is a module
that exists between the <I
CLASS="FIRSTTERM"
>parser stage</I
> and the
<I
CLASS="FIRSTTERM"
>planner/optimizer</I
>. This technique is still implemented.
</P
></LI
></UL
><P>
</P
><P
> The query rewriter is discussed in some detail in
<A
HREF="rules.html"
>Chapter 37</A
>, 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.
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