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<div class="title">Cursor operations </div> </div>
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<div class="textblock"><p>Common operations in WiredTiger are performed using <a class="el" href="struct_w_t___c_u_r_s_o_r.html" title="A WT_CURSOR handle is the interface to a cursor. ">WT_CURSOR</a> handles. A cursor includes:</p>
<ul>
<li>a position within a data source</li>
<li>getter/setters for key and value fields</li>
<li>encoding of fields to store in the data source</li>
<li>methods to navigate within and iterate through the data</li>
</ul>
<h1><a class="anchor" id="cursor_opening"></a>
Opening a cursor</h1>
<p>Cursors are created using the <a class="el" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d" title="Open a new cursor on a data source or duplicate an existing cursor. ">WT_SESSION::open_cursor</a> method. For example, from the program <a class="el" href="ex_cursor_8c-example.html">ex_cursor.c</a>:</p>
<div class="fragment"><div class="line"> error_check(session-><a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(session, <span class="stringliteral">"table:world"</span>, NULL, NULL, &cursor));</div></div><!-- fragment --><p> Another example from the same program:</p>
<div class="fragment"><div class="line"> error_check(</div><div class="line"> session-><a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(session, <span class="stringliteral">"table:world(country,population)"</span>, NULL, NULL, &cursor));</div></div><!-- fragment --><p> In addition to traditional data sources, cursors in WiredTiger are used to access projections and even created data sources such as the run-time statistics:</p>
<div class="fragment"><div class="line"> error_check(session-><a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(session, <span class="stringliteral">"statistics:"</span>, NULL, NULL, &cursor));</div></div><!-- fragment --><p> See <a class="el" href="cursors.html">Cursors</a> for more information on available cursor types.</p>
<h1><a class="anchor" id="cursor_closing"></a>
Closing a cursor</h1>
<p>Cursors remain open until either <a class="el" href="struct_w_t___c_u_r_s_o_r.html#aeea071f192cab12245a50fbe71c3460b" title="Close the cursor. ">WT_CURSOR::close</a> is called or the cursor's session is closed, which may either be in <a class="el" href="struct_w_t___s_e_s_s_i_o_n.html#a96f25dfa6447034aea1f67ab02ab5698" title="Close the session handle. ">WT_SESSION::close</a> or <a class="el" href="struct_w_t___c_o_n_n_e_c_t_i_o_n.html#af535c517df851eeac8ebf3594d40b545" title="Close a connection. ">WT_CONNECTION::close</a>.</p>
<h1><a class="anchor" id="cursor_position"></a>
Positioning a cursor</h1>
<p>Cursors may be positioned at the beginning of the data source, the end of the data source, at an exact key within the data source, and near a key within the data source.</p>
<p>To invalidate the position of a cursor so that subsequent iterations start from the beginning or end of the data source, use the <a class="el" href="struct_w_t___c_u_r_s_o_r.html#afc1b42c22c9c85e1ba08ce3b34437565" title="Reset the cursor. ">WT_CURSOR::reset</a> method:</p>
<div class="fragment"><div class="line"><span class="keywordtype">int</span></div><div class="line">cursor_reset(<a class="code" href="struct_w_t___c_u_r_s_o_r.html">WT_CURSOR</a> *cursor)</div><div class="line">{</div><div class="line"> <span class="keywordflow">return</span> (cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#afc1b42c22c9c85e1ba08ce3b34437565">reset</a>(cursor));</div><div class="line">}</div></div><!-- fragment --><p> To move a cursor forward in the data source, use the cursor <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a0503f16bd8f3d05aa3552f229b3a8e1b" title="Return the next record. ">WT_CURSOR::next</a> method:</p>
<div class="fragment"><div class="line"><span class="keywordtype">int</span></div><div class="line">cursor_forward_scan(<a class="code" href="struct_w_t___c_u_r_s_o_r.html">WT_CURSOR</a> *cursor)</div><div class="line">{</div><div class="line"> <span class="keyword">const</span> <span class="keywordtype">char</span> *key, *value;</div><div class="line"> <span class="keywordtype">int</span> ret;</div><div class="line"></div><div class="line"> <span class="keywordflow">while</span> ((ret = cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a0503f16bd8f3d05aa3552f229b3a8e1b">next</a>(cursor)) == 0) {</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af19f6f9d9c7fc248ab38879032620b2f">get_key</a>(cursor, &key));</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af85364a5af50b95bbc46c82e72f75c01">get_value</a>(cursor, &value));</div><div class="line"> }</div><div class="line"> scan_end_check(ret == <a class="code" href="group__wt.html#ga3c9e1b494d95cf34404ab7a974af6bf8">WT_NOTFOUND</a>);</div><div class="line"></div><div class="line"> <span class="keywordflow">return</span> (0);</div><div class="line">}</div></div><!-- fragment --><p> If the <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a0503f16bd8f3d05aa3552f229b3a8e1b" title="Return the next record. ">WT_CURSOR::next</a> method is called on a cursor without a position in the data source, it is positioned at the beginning of the data source.</p>
<p>To move a cursor backward in the data source, use the cursor <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a43d6664d2f68902aa63f933864242e76" title="Return the previous record. ">WT_CURSOR::prev</a> method:</p>
<div class="fragment"><div class="line"><span class="keywordtype">int</span></div><div class="line">cursor_reverse_scan(<a class="code" href="struct_w_t___c_u_r_s_o_r.html">WT_CURSOR</a> *cursor)</div><div class="line">{</div><div class="line"> <span class="keyword">const</span> <span class="keywordtype">char</span> *key, *value;</div><div class="line"> <span class="keywordtype">int</span> ret;</div><div class="line"></div><div class="line"> <span class="keywordflow">while</span> ((ret = cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a43d6664d2f68902aa63f933864242e76">prev</a>(cursor)) == 0) {</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af19f6f9d9c7fc248ab38879032620b2f">get_key</a>(cursor, &key));</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af85364a5af50b95bbc46c82e72f75c01">get_value</a>(cursor, &value));</div><div class="line"> }</div><div class="line"> scan_end_check(ret == <a class="code" href="group__wt.html#ga3c9e1b494d95cf34404ab7a974af6bf8">WT_NOTFOUND</a>);</div><div class="line"></div><div class="line"> <span class="keywordflow">return</span> (0);</div><div class="line">}</div></div><!-- fragment --><p> If the <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a43d6664d2f68902aa63f933864242e76" title="Return the previous record. ">WT_CURSOR::prev</a> method is called on a cursor without a position in the data source, it is positioned at the end of the data source.</p>
<p>To position a cursor at a specific location in the data source, use the <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a7e25b2ced2cf3ec68bd5429bf921c79f" title="Return the record matching the key. ">WT_CURSOR::search</a> method:</p>
<div class="fragment"><div class="line"><span class="keywordtype">int</span></div><div class="line">cursor_search(<a class="code" href="struct_w_t___c_u_r_s_o_r.html">WT_CURSOR</a> *cursor)</div><div class="line">{</div><div class="line"> <span class="keyword">const</span> <span class="keywordtype">char</span> *value;</div><div class="line"></div><div class="line"> cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#ad1088d719df40babc1f57d086691ebdc">set_key</a>(cursor, <span class="stringliteral">"foo"</span>);</div><div class="line"></div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a7e25b2ced2cf3ec68bd5429bf921c79f">search</a>(cursor));</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af85364a5af50b95bbc46c82e72f75c01">get_value</a>(cursor, &value));</div><div class="line"></div><div class="line"> <span class="keywordflow">return</span> (0);</div><div class="line">}</div></div><!-- fragment --><p> To position a cursor at or near a location in the data source, use the <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a8068ddce20d0775f26f6dac6e5eb209c" title="Return the record matching the key if it exists, or an adjacent record. ">WT_CURSOR::search_near</a> method:</p>
<div class="fragment"><div class="line"><span class="keywordtype">int</span></div><div class="line">cursor_search_near(<a class="code" href="struct_w_t___c_u_r_s_o_r.html">WT_CURSOR</a> *cursor)</div><div class="line">{</div><div class="line"> <span class="keyword">const</span> <span class="keywordtype">char</span> *key, *value;</div><div class="line"> <span class="keywordtype">int</span> exact;</div><div class="line"></div><div class="line"> cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#ad1088d719df40babc1f57d086691ebdc">set_key</a>(cursor, <span class="stringliteral">"foo"</span>);</div><div class="line"></div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a8068ddce20d0775f26f6dac6e5eb209c">search_near</a>(cursor, &exact));</div><div class="line"> <span class="keywordflow">switch</span> (exact) {</div><div class="line"> <span class="keywordflow">case</span> -1: <span class="comment">/* Returned key smaller than search key */</span></div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af19f6f9d9c7fc248ab38879032620b2f">get_key</a>(cursor, &key));</div><div class="line"> <span class="keywordflow">break</span>;</div><div class="line"> <span class="keywordflow">case</span> 0: <span class="comment">/* Exact match found */</span></div><div class="line"> <span class="keywordflow">break</span>;</div><div class="line"> <span class="keywordflow">case</span> 1: <span class="comment">/* Returned key larger than search key */</span></div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af19f6f9d9c7fc248ab38879032620b2f">get_key</a>(cursor, &key));</div><div class="line"> <span class="keywordflow">break</span>;</div><div class="line"> }</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af85364a5af50b95bbc46c82e72f75c01">get_value</a>(cursor, &value));</div><div class="line"></div><div class="line"> <span class="keywordflow">return</span> (0);</div><div class="line">}</div></div><!-- fragment --><p> Cursor positions do not survive transactions: cursors that are open during <a class="el" href="struct_w_t___s_e_s_s_i_o_n.html#a7e26b16b26b5870498752322fad790bf" title="Start a transaction in this session. ">WT_SESSION::begin_transaction</a>, <a class="el" href="struct_w_t___s_e_s_s_i_o_n.html#a712226eca5ade5bd123026c624468fa2" title="Commit the current transaction. ">WT_SESSION::commit_transaction</a> or <a class="el" href="struct_w_t___s_e_s_s_i_o_n.html#ab45f521464ad9e54d9b15efc2ffe20a1" title="Roll back the current transaction. ">WT_SESSION::rollback_transaction</a> will lose their position as if <a class="el" href="struct_w_t___c_u_r_s_o_r.html#afc1b42c22c9c85e1ba08ce3b34437565" title="Reset the cursor. ">WT_CURSOR::reset</a> was called.</p>
<p>Cursors can be configured to move to a random position with <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a0503f16bd8f3d05aa3552f229b3a8e1b" title="Return the next record. ">WT_CURSOR::next</a> is called, see <a class="el" href="cursor_random.html">Cursor random</a> for details.</p>
<h1><a class="anchor" id="cursor_writes"></a>
Inserting and updating</h1>
<p>To insert new data, and optionally update existing data, using a cursor, use the <a class="el" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3" title="Insert a record and optionally update an existing record. ">WT_CURSOR::insert</a> method:</p>
<div class="fragment"><div class="line"><span class="keywordtype">int</span></div><div class="line">cursor_insert(<a class="code" href="struct_w_t___c_u_r_s_o_r.html">WT_CURSOR</a> *cursor)</div><div class="line">{</div><div class="line"> cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#ad1088d719df40babc1f57d086691ebdc">set_key</a>(cursor, <span class="stringliteral">"foo"</span>);</div><div class="line"> cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a27f7cbd0cd3e561f6a145704813ad64c">set_value</a>(cursor, <span class="stringliteral">"bar"</span>);</div><div class="line"></div><div class="line"> <span class="keywordflow">return</span> (cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3">insert</a>(cursor));</div><div class="line">}</div></div><!-- fragment --><p> To update existing data using a cursor, use the <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a444cdc0952e7f8d55d23173516c7037f" title="Update an existing record and optionally insert a record. ">WT_CURSOR::update</a> method:</p>
<div class="fragment"><div class="line"><span class="keywordtype">int</span></div><div class="line">cursor_update(<a class="code" href="struct_w_t___c_u_r_s_o_r.html">WT_CURSOR</a> *cursor)</div><div class="line">{</div><div class="line"> cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#ad1088d719df40babc1f57d086691ebdc">set_key</a>(cursor, <span class="stringliteral">"foo"</span>);</div><div class="line"> cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a27f7cbd0cd3e561f6a145704813ad64c">set_value</a>(cursor, <span class="stringliteral">"newbar"</span>);</div><div class="line"></div><div class="line"> <span class="keywordflow">return</span> (cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a444cdc0952e7f8d55d23173516c7037f">update</a>(cursor));</div><div class="line">}</div></div><!-- fragment --><p> To remove existing data using a cursor, use the <a class="el" href="struct_w_t___c_u_r_s_o_r.html#abbba24fe607fee519c4c9c4669cd4455" title="Remove a record. ">WT_CURSOR::remove</a> method:</p>
<div class="fragment"><div class="line"><span class="keywordtype">int</span></div><div class="line">cursor_remove(<a class="code" href="struct_w_t___c_u_r_s_o_r.html">WT_CURSOR</a> *cursor)</div><div class="line">{</div><div class="line"> cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#ad1088d719df40babc1f57d086691ebdc">set_key</a>(cursor, <span class="stringliteral">"foo"</span>);</div><div class="line"> <span class="keywordflow">return</span> (cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#abbba24fe607fee519c4c9c4669cd4455">remove</a>(cursor));</div><div class="line">}</div></div><!-- fragment --><p> The <a class="el" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d" title="Open a new cursor on a data source or duplicate an existing cursor. ">WT_SESSION::open_cursor</a> <code>overwrite</code> configuration is <code>true</code> by default, causing <a class="el" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3" title="Insert a record and optionally update an existing record. ">WT_CURSOR::insert</a>, <a class="el" href="struct_w_t___c_u_r_s_o_r.html#abbba24fe607fee519c4c9c4669cd4455" title="Remove a record. ">WT_CURSOR::remove</a> and <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a444cdc0952e7f8d55d23173516c7037f" title="Update an existing record and optionally insert a record. ">WT_CURSOR::update</a> to ignore the current state of the record, and these methods will succeed regardless of whether or not the record previously exists.</p>
<p>When an application configures <code>overwrite</code> to <code>false</code>, <a class="el" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3" title="Insert a record and optionally update an existing record. ">WT_CURSOR::insert</a> will fail with <a class="el" href="group__wt.html#gaeca1f3c4a70b8b71b56d2c2fc99a469c" title="Attempt to insert an existing key. ">WT_DUPLICATE_KEY</a> if the record previously exists, and <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a444cdc0952e7f8d55d23173516c7037f" title="Update an existing record and optionally insert a record. ">WT_CURSOR::update</a> and <a class="el" href="struct_w_t___c_u_r_s_o_r.html#abbba24fe607fee519c4c9c4669cd4455" title="Remove a record. ">WT_CURSOR::remove</a> will fail with <a class="el" href="group__wt.html#ga3c9e1b494d95cf34404ab7a974af6bf8" title="Item not found. ">WT_NOTFOUND</a> if the record does not previously exist.</p>
<h1><a class="anchor" id="cursor_error"></a>
Cursor position after error</h1>
<p>After any cursor handle method failure, the cursor's position is undetermined. For cursor operations that expect a key to be set before the operation begins (including <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a7e25b2ced2cf3ec68bd5429bf921c79f" title="Return the record matching the key. ">WT_CURSOR::search</a>, <a class="el" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3" title="Insert a record and optionally update an existing record. ">WT_CURSOR::insert</a>, <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a444cdc0952e7f8d55d23173516c7037f" title="Update an existing record and optionally insert a record. ">WT_CURSOR::update</a> and <a class="el" href="struct_w_t___c_u_r_s_o_r.html#abbba24fe607fee519c4c9c4669cd4455" title="Remove a record. ">WT_CURSOR::remove</a>), the application's key and value will not be cleared by an error.</p>
<p>Applications that cannot re-position the cursor after failure must duplicate the cursor by calling <a class="el" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d" title="Open a new cursor on a data source or duplicate an existing cursor. ">WT_SESSION::open_cursor</a> and passing the cursor as the <code>to_dup</code> parameter before calling a cursor method that will attempt to re-position the cursor. Cursor duplication is not supported for the backup, config and statistics cursor types.</p>
<h1><a class="anchor" id="cursor_memory_scoping"></a>
Cursor key/value memory scoping</h1>
<p>When applications pass a pointer (either to a <a class="el" href="group__wt.html#struct_w_t___i_t_e_m" title="A raw item of data to be managed, including a pointer to the data and a length. ">WT_ITEM</a> or a string), to <a class="el" href="struct_w_t___c_u_r_s_o_r.html#ad1088d719df40babc1f57d086691ebdc" title="Set the key for the next operation. ">WT_CURSOR::set_key</a> or <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a27f7cbd0cd3e561f6a145704813ad64c" title="Set the value for the next operation. ">WT_CURSOR::set_value</a>, WiredTiger does not copy the memory referenced by the pointer. For this reason, the application must keep the referenced memory unchanged and valid until the next operation that successfully positions the cursor, modifies the underlying data, or the cursor is reset or closed (discarding its resources). The operations that position the cursor are <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a0503f16bd8f3d05aa3552f229b3a8e1b" title="Return the next record. ">WT_CURSOR::next</a>, <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a43d6664d2f68902aa63f933864242e76" title="Return the previous record. ">WT_CURSOR::prev</a>, <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a7e25b2ced2cf3ec68bd5429bf921c79f" title="Return the record matching the key. ">WT_CURSOR::search</a> and <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a8068ddce20d0775f26f6dac6e5eb209c" title="Return the record matching the key if it exists, or an adjacent record. ">WT_CURSOR::search_near</a>; the operations that modify the underlying data are <a class="el" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3" title="Insert a record and optionally update an existing record. ">WT_CURSOR::insert</a>, <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a444cdc0952e7f8d55d23173516c7037f" title="Update an existing record and optionally insert a record. ">WT_CURSOR::update</a> and <a class="el" href="struct_w_t___c_u_r_s_o_r.html#abbba24fe607fee519c4c9c4669cd4455" title="Remove a record. ">WT_CURSOR::remove</a>.</p>
<p>If a cursor operation fails (for example, due to a <a class="el" href="group__wt.html#ga5ee3c6fcd074e11efd118f3e68e91db8" title="Conflict between concurrent operations. ">WT_ROLLBACK</a> error), it may be retried without calling <a class="el" href="struct_w_t___c_u_r_s_o_r.html#ad1088d719df40babc1f57d086691ebdc" title="Set the key for the next operation. ">WT_CURSOR::set_key</a> or <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a27f7cbd0cd3e561f6a145704813ad64c" title="Set the value for the next operation. ">WT_CURSOR::set_value</a> again. That is, the cursor may still reference the application-supplied memory until the cursor is successfully positioned, underlying data is modified, or the cursor is closed or reset.</p>
<p>Any pointers returned by <a class="el" href="struct_w_t___c_u_r_s_o_r.html#af19f6f9d9c7fc248ab38879032620b2f" title="Get the key for the current record. ">WT_CURSOR::get_key</a> or <a class="el" href="struct_w_t___c_u_r_s_o_r.html#af85364a5af50b95bbc46c82e72f75c01" title="Get the value for the current record. ">WT_CURSOR::get_value</a> are only valid until a subsequent cursor call that successfully positions the cursor, modifies the underlying data, or the cursor is reset or closed. These pointers may reference private WiredTiger data structures that may not be modified or freed by the application. If a longer scope is required, the application must make a copy of the memory before the cursor is re-used, closed or reset. </p>
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