<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<title>Introduction</title>
</head>
<body><div class="manualnavbar" style="text-align: center;">
<div class="prev" style="text-align: left; float: left;"><a href="book.chdb.html">chdb</a></div>
<div class="next" style="text-align: right; float: right;"><a href="chdb.setup.html">Installing/Configuring</a></div>
<div class="up"><a href="book.chdb.html">chdb</a></div>
<div class="home"><a href="index.html">PHP Manual</a></div>
</div><hr /><div id="intro.chdb" class="preface">
<h1 class="title">Introduction</h1>
<p class="para">
CHDB (constant hash database) is a fast key-value database for constant data,
realized by using a memory-mapped file and thus providing the following
functionalities:
<ul class="itemizedlist">
<li class="listitem">
<p class="para">
Extremely fast initial load, regardless of the size of the database.
</p>
</li>
<li class="listitem">
<p class="para">
Only the pages of the file which are actually used are loaded from the
disk.
</p>
</li>
<li class="listitem">
<p class="para">
Once a page is loaded it is shared across multiple processes.
</p>
</li>
<li class="listitem">
<p class="para">
Loaded pages are cached across multiple requests and even process recycling.
</p>
</li>
</ul>
</p>
<p class="para">
A typical use of CHDB is as a faster alternative to defining many PHP
constants.
</p>
<p class="para">
CHDB is internally implemented as a hash-table using a
<a href="http://en.wikipedia.org/wiki/Perfect_hash_function" class="link external">» perfect hashing</a>
function, thus guaranteeing worst case O(1) lookup time.
</p>
</div><hr /><div class="manualnavbar" style="text-align: center;">
<div class="prev" style="text-align: left; float: left;"><a href="book.chdb.html">chdb</a></div>
<div class="next" style="text-align: right; float: right;"><a href="chdb.setup.html">Installing/Configuring</a></div>
<div class="up"><a href="book.chdb.html">chdb</a></div>
<div class="home"><a href="index.html">PHP Manual</a></div>
</div></body></html>
