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diff --git a/www/inmemorydb.html b/www/inmemorydb.html new file mode 100644 index 0000000..df4e732 --- /dev/null +++ b/www/inmemorydb.html @@ -0,0 +1,252 @@ +<!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>In-Memory Databases</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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However, in certain circumstances, the database might be stored in +memory.</p> + +<p>The most common way to force an SQLite database to exist purely +in memory is to open the database using the special filename +"<b>:memory:</b>". In other words, instead of passing the name of +a real disk file into one of the <a href="c3ref/open.html">sqlite3_open()</a>, <a href="c3ref/open.html">sqlite3_open16()</a>, or +<a href="c3ref/open.html">sqlite3_open_v2()</a> functions, pass in the string ":memory:". For +example:</p> + +<blockquote><pre> +rc = sqlite3_open(":memory:", &db); +</pre></blockquote> + +<p>When this is done, no disk file is opened. +Instead, a new database is created +purely in memory. The database ceases to exist as soon as the database +connection is closed. Every :memory: database is distinct from every +other. So, opening two database connections each with the filename +":memory:" will create two independent in-memory databases.</p> + +<p>The special filename ":memory:" can be used anywhere that a database +filename is permitted. For example, it can be used as the +<i>filename</i> in an <a href="lang_attach.html">ATTACH</a> command:</p> + +<blockquote><pre> +ATTACH DATABASE ':memory:' AS aux1; +</pre></blockquote> + +<p>Note that in order for the special ":memory:" name to apply and to +create a pure in-memory database, there must be no additional text in the +filename. Thus, a disk-based database can be created in a file by prepending +a pathname, like this: "./:memory:".</p> + +<p>The special ":memory:" filename also works when using <a href="uri.html">URI filenames</a>. +For example: + +<blockquote><pre> +rc = sqlite3_open("file::memory:", &db); +</pre></blockquote> + +Or, + +<blockquote><pre> +ATTACH DATABASE 'file::memory:' AS aux1; +</pre></blockquote> + +<a name="sharedmemdb"></a> + +<h2>In-memory Databases And Shared Cache</h2> + +<p>In-memory databases are allowed to use <a href="sharedcache.html">shared cache</a> if they are +opened using a <a href="uri.html">URI filename</a>. If the unadorned ":memory:" name is used +to specify the in-memory database, then that database always has a private +cache and is only visible to the database connection that originally +opened it. However, the same in-memory database can be opened by two or +more database connections as follows: + +<blockquote><pre> +rc = sqlite3_open("file::memory:?cache=shared", &db); +</pre></blockquote> + +Or, + +<blockquote><pre> +ATTACH DATABASE 'file::memory:?cache=shared' AS aux1; +</pre></blockquote> + +<p>This allows separate database connections to share the same +in-memory database. Of course, all database connections sharing the +in-memory database need to be in the same process. The database is +automatically deleted and memory is reclaimed when the last connection +to the database closes. + +<p>If two or more distinct but shareable in-memory databases are needed +in a single process, then the <a href="uri.html#coreqp">mode=memory</a> query parameter can +be used with a <a href="uri.html">URI filename</a> to create a named in-memory database: + +<blockquote><pre> +rc = sqlite3_open("file:memdb1?mode=memory&cache=shared", &db); +</pre></blockquote> + +Or, + +<blockquote><pre> +ATTACH DATABASE 'file:memdb1?mode=memory&cache=shared' AS aux1; +</pre></blockquote> + +<p>When an in-memory database is named in this way, it will only share its +cache with another connection that uses exactly the same name. + + + +<a name="temp_db"></a> + +<h2>Temporary Databases</h2> + +<p>When the name of the database file handed to <a href="c3ref/open.html">sqlite3_open()</a> or to +<a href="lang_attach.html">ATTACH</a> is an empty string, then a new temporary file is created to hold +the database.</p> + +<blockquote><pre> +rc = sqlite3_open("", &db); +</pre></blockquote> + +<blockquote><pre> +ATTACH DATABASE '' AS aux2; +</pre></blockquote> + +<p>A different temporary file is created each time so that, just as +with the special ":memory:" string, two database connections to temporary +databases each have their own private database. Temporary databases are +automatically deleted when the connection that created them closes.</p> + +<p>Even though a disk file is allocated for each temporary database, in +practice the temporary database usually resides in the in-memory pager cache +and hence there is very little difference between a pure in-memory database +created by ":memory:" and a temporary database created by an empty filename. +The sole difference is that a ":memory:" database must remain in memory +at all times whereas parts of a temporary database might be flushed to disk +if the database becomes large or if SQLite comes under memory pressure.</p> + +<p>The previous paragraphs describe the behavior of temporary databases +under the default SQLite configuration. An application can use the +<a href="pragma.html#pragma_temp_store">temp_store pragma</a> and the <a href="compile.html#temp_store">SQLITE_TEMP_STORE</a> compile-time parameter to +force temporary databases to behave as pure in-memory databases, if desired. +</p> +<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/inmemorydb.in?m=2e26d2c7a8">2022-04-18 02:55:50</a> UTC </small></i></p> + |