149 lines
6.0 KiB
HTML
149 lines
6.0 KiB
HTML
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<a href="intro.html"><h2>SQLite C Interface</h2></a>
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<h2>Start a read transaction on an historical snapshot</h2>
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</div>
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<blockquote><pre>
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int sqlite3_snapshot_open(
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sqlite3 *db,
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const char *zSchema,
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sqlite3_snapshot *pSnapshot
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);
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</pre></blockquote>
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<p>
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The <a href="../c3ref/snapshot_open.html">sqlite3_snapshot_open(D,S,P)</a> interface either starts a new read
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transaction or upgrades an existing one for schema S of
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<a href="../c3ref/sqlite3.html">database connection</a> D such that the read transaction refers to
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historical <a href="../c3ref/snapshot.html">snapshot</a> P, rather than the most recent change to the
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database. The <a href="../c3ref/snapshot_open.html">sqlite3_snapshot_open()</a> interface returns SQLITE_OK
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on success or an appropriate <a href="../rescode.html">error code</a> if it fails.</p>
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<p>In order to succeed, the database connection must not be in
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<a href="../c3ref/get_autocommit.html">autocommit mode</a> when <a href="../c3ref/snapshot_open.html">sqlite3_snapshot_open(D,S,P)</a> is called. If there
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is already a read transaction open on schema S, then the database handle
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must have no active statements (SELECT statements that have been passed
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to sqlite3_step() but not sqlite3_reset() or sqlite3_finalize()).
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SQLITE_ERROR is returned if either of these conditions is violated, or
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if schema S does not exist, or if the snapshot object is invalid.</p>
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<p>A call to sqlite3_snapshot_open() will fail to open if the specified
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snapshot has been overwritten by a <a href="../wal.html#ckpt">checkpoint</a>. In this case
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SQLITE_ERROR_SNAPSHOT is returned.</p>
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<p>If there is already a read transaction open when this function is
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invoked, then the same read transaction remains open (on the same
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database snapshot) if SQLITE_ERROR, SQLITE_BUSY or SQLITE_ERROR_SNAPSHOT
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is returned. If another error code - for example SQLITE_PROTOCOL or an
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SQLITE_IOERR error code - is returned, then the final state of the
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read transaction is undefined. If SQLITE_OK is returned, then the
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read transaction is now open on database snapshot P.</p>
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<p>A call to <a href="../c3ref/snapshot_open.html">sqlite3_snapshot_open(D,S,P)</a> will fail if the
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database connection D does not know that the database file for
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schema S is in <a href="../wal.html">WAL mode</a>. A database connection might not know
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that the database file is in <a href="../wal.html">WAL mode</a> if there has been no prior
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I/O on that database connection, or if the database entered <a href="../wal.html">WAL mode</a>
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after the most recent I/O on the database connection.
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(Hint: Run "<a href="../pragma.html#pragma_application_id">PRAGMA application_id</a>" against a newly opened
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database connection in order to make it ready to use snapshots.)</p>
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<p>The <a href="../c3ref/snapshot_open.html">sqlite3_snapshot_open()</a> interface is only available when the
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<a href="../compile.html#enable_snapshot">SQLITE_ENABLE_SNAPSHOT</a> compile-time option is used.
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</p><p>See also lists of
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<a href="objlist.html">Objects</a>,
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<a href="constlist.html">Constants</a>, and
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<a href="funclist.html">Functions</a>.</p>
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