<!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>