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diff --git a/doc/src/sgml/html/query-path.html b/doc/src/sgml/html/query-path.html new file mode 100644 index 0000000..e8efce7 --- /dev/null +++ b/doc/src/sgml/html/query-path.html @@ -0,0 +1,55 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>52.1. The Path of a Query</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="overview.html" title="Chapter 52. Overview of PostgreSQL Internals" /><link rel="next" href="connect-estab.html" title="52.2. How Connections Are Established" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">52.1. The Path of a Query</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="overview.html" title="Chapter 52. Overview of PostgreSQL Internals">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="overview.html" title="Chapter 52. Overview of PostgreSQL Internals">Up</a></td><th width="60%" align="center">Chapter 52. Overview of PostgreSQL Internals</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="connect-estab.html" title="52.2. How Connections Are Established">Next</a></td></tr></table><hr /></div><div class="sect1" id="QUERY-PATH"><div class="titlepage"><div><div><h2 class="title" style="clear: both">52.1. The Path of a Query</h2></div></div></div><p> + Here we give a short overview of the stages a query has to pass + to obtain a result. + </p><div class="procedure"><ol class="procedure" 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 <em class="firstterm">parser stage</em> checks the query + transmitted by the application + program for correct syntax and creates + a <em class="firstterm">query tree</em>. + </p></li><li class="step"><p> + The <em class="firstterm">rewrite system</em> takes + the query tree created by the parser stage and looks for + any <em class="firstterm">rules</em> (stored in the + <em class="firstterm">system catalogs</em>) to apply to + the query tree. It performs the + transformations given in the <em class="firstterm">rule bodies</em>. + </p><p> + One application of the rewrite system is in the realization of + <em class="firstterm">views</em>. + Whenever a query against a view + (i.e., a <em class="firstterm">virtual table</em>) is made, + the rewrite system rewrites the user's query to + a query that accesses the <em class="firstterm">base tables</em> given in + the <em class="firstterm">view definition</em> instead. + </p></li><li class="step"><p> + The <em class="firstterm">planner/optimizer</em> takes + the (rewritten) query tree and creates a + <em class="firstterm">query plan</em> that will be the input to the + <em class="firstterm">executor</em>. + </p><p> + It does so by first creating all possible <em class="firstterm">paths</em> + 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 <em class="firstterm">plan tree</em> and + retrieves rows in the way represented by the plan. + The executor makes use of the + <em class="firstterm">storage system</em> while scanning + relations, performs <em class="firstterm">sorts</em> and <em class="firstterm">joins</em>, + evaluates <em class="firstterm">qualifications</em> 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. + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="overview.html" title="Chapter 52. Overview of PostgreSQL Internals">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="overview.html" title="Chapter 52. Overview of PostgreSQL Internals">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="connect-estab.html" title="52.2. How Connections Are Established">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 52. Overview of PostgreSQL Internals </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 52.2. How Connections Are Established</td></tr></table></div></body></html>
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