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diff --git a/www/dbpage.html b/www/dbpage.html new file mode 100644 index 0000000..77916bb --- /dev/null +++ b/www/dbpage.html @@ -0,0 +1,236 @@ +<!DOCTYPE html> +<html><head> +<meta name="viewport" content="width=device-width, initial-scale=1.0"> +<meta http-equiv="content-type" content="text/html; charset=UTF-8"> +<link href="sqlite.css" rel="stylesheet"> +<title>The SQLITE_DBPAGE Virtual Table</title> +<!-- path= --> +</head> +<body> +<div class=nosearch> +<a href="index.html"> +<img class="logo" src="images/sqlite370_banner.gif" alt="SQLite" border="0"> +</a> +<div><!-- IE hack to prevent disappearing logo --></div> +<div class="tagline desktoponly"> +Small. Fast. 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SQLITE_DBPAGE is capable of both reading and writing any +page of the database. Because interaction is through the pager layer, all +changes are transactional. +</p> + +<p> +<b>Warning:</b> writing to the SQLITE_DBPAGE virtual table can very easily +cause unrecoverably database corruption. Do not allow untrusted components +to access the SQLITE_DBPAGE table. Use appropriate care while using the +SQLITE_DBPAGE table. Back up important data prior to experimenting with the +SQLITE_DBPAGE table. Writes to the SQLITE_DBPAGE virtual table are +disabled when the <a href="c3ref/c_dbconfig_defensive.html#sqlitedbconfigdefensive">SQLITE_DBCONFIG_DEFENSIVE</a> flag is set. + +</p><p> +The SQLITE_DBPAGE extension is included in the <a href="amalgamation.html">amalgamation</a> though +it is disabled +by default. Use the <a href="compile.html#enable_dbpage_vtab">SQLITE_ENABLE_DBPAGE_VTAB</a> compile-time option to enable +the SQLITE_DBPAGE extension. The SQLITE_DBPAGE extension makes use of +unpublished internal interfaces and is not run-time loadable. The only way +to add SQLITE_DBPAGE to an application is to compile it in using the +<a href="compile.html#enable_dbpage_vtab">SQLITE_ENABLE_DBPAGE_VTAB</a> compile-time option. +</p> + +<p> +The SQLITE_DBPAGE extension is enabled in default builds +of the <a href="cli.html">command-line shell</a>. + +</p><h1 id="usage"><span>2. </span>Usage</h1> + +<p> +The SQLITE_DBPAGE virtual table read/write table that provides direct +access to the underlying disk file on a page-by-page basis. The +virtual table appears to have a schema like this: + +</p><div class="codeblock"><pre>CREATE TABLE sqlite_dbpage( + pgno INTEGER PRIMARY KEY, + data BLOB +); +</pre></div> + +<p> +An SQLite database file is divided into pages. +The first page is 1, the second page is 2, and so forth. +There is no page 0. +Every page is the same size. +The size of every page is a power of 2 between 512 and 65536. +See the <a href="fileformat2.html">file format</a> documentation for further details. + +</p><p> +The SQLITE_DBPAGE table allows an application to view or replace the +raw binary content of each page of the database file. +No attempt is made to interpret the content of the page. +Content is returned byte-for-byte as it appears on disk. + +</p><p> +The SQLITE_DBPAGE table has one row for each page in the database file. +SQLITE_DBPAGE allows pages to be read or to be overwritten. +However the size of the database file cannot be changed. It is not +possible to change the number of rows in the SQLITE_DBPAGE table by +running DELETE or INSERT operations against that table. + +</p><h2 id="using_sqlite_dbpage_on_attach_ed_databases"><span>2.1. </span>Using SQLITE_DBPAGE On ATTACH-ed Databases</h2> + +<p> +The SQLITE_DBPAGE table schema shown above is incomplete. There is +a third <a href="vtab.html#hiddencol">hidden column</a> named "schema" that determines which +<a href="lang_attach.html">ATTACH-ed database</a> should be read or written. Because +the "schema" column is hidden, it can be used as a parameter when +SQLITE_DBPAGE is invoked as a <a href="vtab.html#tabfunc2">table-valued function</a>. + +</p><p> +For example, suppose an additional database is attached to the +database connection using a statement like this: + +</p><div class="codeblock"><pre>ATTACH 'auxdata1.db' AS aux1; +</pre></div> + +<p> +Then to read the first page of that database file, one merely runs: + +</p><div class="codeblock"><pre>SELECT data FROM sqlite_dbpage('aux1') WHERE pgno=1; +</pre></div> + +<p> +If the "schema" is omitted, it defaults to the primary database +(usually called 'main', unless renamed using <a href="c3ref/c_dbconfig_defensive.html#sqlitedbconfigmaindbname">SQLITE_DBCONFIG_MAINDBNAME</a>). +Hence, the following two queries are normally equivalent: + +</p><div class="codeblock"><pre>SELECT data FROM sqlite_dbpage('main') WHERE pgno=1; +SELECT data FROM sqlite_dbpage WHERE pgno=1; +</pre></div> + +<p> +The SQLITE_DBPAGE table can participate in a join just like any other +table. Hence, to see the content of the first page to all connected +database files, one might run a statement like this: + +</p><div class="codeblock"><pre>SELECT dbpage.data, dblist.name + FROM pragma_database_list AS dblist + JOIN sqlite_dbpage(dblist.name) AS dbpage + WHERE dbpage.pgno=1; +</pre></div> +<p align="center"><small><i>This page last modified on <a href="https://sqlite.org/docsrc/honeypot" id="mtimelink" data-href="https://sqlite.org/docsrc/finfo/pages/dbpage.in?m=21798cdb09">2022-01-08 05:02:57</a> UTC </small></i></p> + |