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>The Path of a Query</TITLE
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>44.1. The Path of a Query</A
></H1
><P
> Here we give a short overview of the stages a query has to pass in
order to obtain a result.
</P
><DIV
CLASS="PROCEDURE"
><OL
TYPE="1"
><LI
CLASS="STEP"
><P
> A connection from an application program to the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
server has to be established. The application program transmits a
query to the server and waits to receive the results sent back by the
server.
</P
></LI
><LI
CLASS="STEP"
><P
> The <I
CLASS="FIRSTTERM"
>parser stage</I
> checks the query
transmitted by the application
program for correct syntax and creates
a <I
CLASS="FIRSTTERM"
>query tree</I
>.
</P
></LI
><LI
CLASS="STEP"
><P
> The <I
CLASS="FIRSTTERM"
>rewrite system</I
> takes
the query tree created by the parser stage and looks for
any <I
CLASS="FIRSTTERM"
>rules</I
> (stored in the
<I
CLASS="FIRSTTERM"
>system catalogs</I
>) to apply to
the query tree. It performs the
transformations given in the <I
CLASS="FIRSTTERM"
>rule bodies</I
>.
</P
><P
> One application of the rewrite system is in the realization of
<I
CLASS="FIRSTTERM"
>views</I
>.
Whenever a query against a view
(i.e., a <I
CLASS="FIRSTTERM"
>virtual table</I
>) is made,
the rewrite system rewrites the user's query to
a query that accesses the <I
CLASS="FIRSTTERM"
>base tables</I
> given in
the <I
CLASS="FIRSTTERM"
>view definition</I
> instead.
</P
></LI
><LI
CLASS="STEP"
><P
> The <I
CLASS="FIRSTTERM"
>planner/optimizer</I
> takes
the (rewritten) query tree and creates a
<I
CLASS="FIRSTTERM"
>query plan</I
> that will be the input to the
<I
CLASS="FIRSTTERM"
>executor</I
>.
</P
><P
> It does so by first creating all possible <I
CLASS="FIRSTTERM"
>paths</I
>
leading to the same result. For example if there is an index on a
relation to be scanned, there are two paths for the
scan. One possibility is a simple sequential scan and the other
possibility is to use the index. Next the cost for the execution of
each path is estimated and the cheapest path is chosen. The cheapest
path is expanded into a complete plan that the executor can use.
</P
></LI
><LI
CLASS="STEP"
><P
> The executor recursively steps through
the <I
CLASS="FIRSTTERM"
>plan tree</I
> and
retrieves rows in the way represented by the plan.
The executor makes use of the
<I
CLASS="FIRSTTERM"
>storage system</I
> while scanning
relations, performs <I
CLASS="FIRSTTERM"
>sorts</I
> and <I
CLASS="FIRSTTERM"
>joins</I
>,
evaluates <I
CLASS="FIRSTTERM"
>qualifications</I
> and finally hands back the rows derived.
</P
></LI
></OL
></DIV
><P
> In the following sections we will cover each of the above listed items
in more detail to give a better understanding of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s internal
control and data structures.
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