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<div class="title">Schema, Columns, Column Groups, Indices and Projections in Java </div> </div>
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<div class="textblock"><p>While many tables have simple key/value pairs for records, WiredTiger also supports more complex data patterns.</p>
<h1><a class="anchor" id="schema_intro_lang_java"></a>
Tables, rows and columns</h1>
<p>A table is a logical representation of data consisting of cells in rows and columns. For example, a database might have a simple table including an employee identifier, last name and first name, and a salary:</p>
<table class="doxtable">
<tr>
<th>Employee ID</th><th>Last Name</th><th>First Name</th><th>Salary </th></tr>
<tr>
<td>1</td><td>Smith</td><td>Joe</td><td>40000 </td></tr>
<tr>
<td>2</td><td>Jones</td><td>Mary</td><td>50000 </td></tr>
<tr>
<td>3</td><td>Johnson</td><td>Cathy</td><td>44000 </td></tr>
</table>
<p>A row-oriented database would store all of the values for the first employee in the first row, then the next employee's values in the next row, and so on:</p>
<pre>
1,Smith,Joe,40000
2,Jones,Mary,50000
3,Johnson,Cathy,44000
</pre><p>A column-oriented database would store all of the values of a column together, then the values of the next column, and so on:</p>
<pre>
1,2,3
Smith,Jones,Johnson
Joe,Mary,Cathy
40000,50000,44000
</pre><p>WiredTiger supports both storage formats, and can mix and match the storage of columns within a logical table.</p>
<p>A <em>table</em> in WiredTiger consist of one or more <em>"column groups"</em> that together hold all of the columns in primary key order; and zero or more <em>indices</em> that enable fast lookup of records by columns in orders other than the primary key.</p>
<p>Applications describe the format of their data by supplying a schema to Session.create. This specifies how the application's data can be split into fields and mapped onto rows and columns.</p>
<h1><a class="anchor" id="schema_column_types_lang_java"></a>
Column types</h1>
<p>By default, WiredTiger works as a traditional key/value store, where the keys and values are raw byte arrays accessed using 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> structure. Key and value types may also be chosen from a list, or composed of multiple columns with any combination of types. Keys and values may be up to (<code>4GB - 512B</code>) bytes in size.</p>
<p>See <a class="el" href="keyvalue.html">Key/Value pairs</a> for more details on raw key / value items.</p>
<h1><a class="anchor" id="schema_format_types_lang_java"></a>
Format types</h1>
<p>WiredTiger's uses format strings similar to those specified in the Python struct module to describe the types of columns in a table: <a href="http://docs.python.org/library/struct">http://docs.python.org/library/struct</a></p>
<table class="doxtable">
<tr>
<th>Format</th><th>C Type</th><th>Java type</th><th>Python type</th><th>Notes </th></tr>
<tr>
<td><code>x</code> </td><td>N/A</td><td>N/A</td><td>N/A</td><td>pad byte, no associated value </td></tr>
<tr>
<td><code>b</code> </td><td><code>int8_t</code> </td><td><code>byte</code> </td><td><code>int</code> </td><td>signed byte </td></tr>
<tr>
<td><code>B</code> </td><td><code>uint8_t</code> </td><td><code>byte</code> </td><td><code>int</code> </td><td>unsigned byte </td></tr>
<tr>
<td><code>h</code> </td><td><code>int16_t</code> </td><td><code>short</code> </td><td><code>int</code> </td><td>signed 16-bit </td></tr>
<tr>
<td><code>H</code> </td><td><code>uint16_t</code> </td><td><code>short</code> </td><td><code>int</code> </td><td>unsigned 16-bit </td></tr>
<tr>
<td><code>i</code> </td><td><code>int32_t</code> </td><td><code>int</code> </td><td><code>int</code> </td><td>signed 32-bit </td></tr>
<tr>
<td><code>I</code> </td><td><code>uint32_t</code> </td><td><code>int</code> </td><td><code>int</code> </td><td>unsigned 32-bit </td></tr>
<tr>
<td><code>l</code> </td><td><code>int32_t</code> </td><td><code>int</code> </td><td><code>int</code> </td><td>signed 32-bit </td></tr>
<tr>
<td><code>L</code> </td><td><code>uint32_t</code> </td><td><code>int</code> </td><td><code>int</code> </td><td>unsigned 32-bit </td></tr>
<tr>
<td><code>q</code> </td><td><code>int64_t</code> </td><td><code>long</code> </td><td><code>int</code> </td><td>signed 64-bit </td></tr>
<tr>
<td><code>Q</code> </td><td><code>uint64_t</code> </td><td><code>long</code> </td><td><code>int</code> </td><td>unsigned 64-bit </td></tr>
<tr>
<td><code>r</code> </td><td><code>uint64_t</code> </td><td><code>long</code> </td><td><code>int</code> </td><td>record number </td></tr>
<tr>
<td><code>s</code> </td><td><code>char</code>[]</td><td><code>String</code> </td><td><code>str</code> </td><td>fixed-length string </td></tr>
<tr>
<td><code>S</code> </td><td><code>char</code>[]</td><td><code>String</code> </td><td><code>str</code> </td><td>NUL-terminated string </td></tr>
<tr>
<td><code>t</code> </td><td><code>uint8_t</code> </td><td><code>byte</code> </td><td><code>int</code> </td><td>fixed-length bit field </td></tr>
<tr>
<td><code>u</code> </td><td><code><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> *</code></td><td><code>byte[]</code></td><td><code>str</code> </td><td>raw byte array </td></tr>
</table>
<p>The <code>'r'</code> type is used for record number keys in column stores. It is otherwise identical to the <code>'Q'</code> type.</p>
<p>The <code>'s'</code> type is used for fixed-length strings. If it is preceded by a size, that indicates the number of bytes to store; the default is a length of 1 byte.</p>
<p>The <code>'S'</code> type is encoded as a C language string terminated by a NUL character. When preceded by a size, that indicates the maximum number of bytes the string can store. In a string with characters less than the specified size, the remaining bytes are NUL padded. If the supplied string is longer than the specified size, it will be stored without a trailing NUL. Because of this, the associated Java String may not contain the NUL character.</p>
<p>The <code>'t'</code> type is used for fixed-length bit field values. If it is preceded by a size, that indicates the number of bits to store, between 1 and 8. That number of low-order bits will be stored in the table. The default is a size of 1 bit: that is, a boolean. If a bit field value is combined with other types in a packing format, it is equivalent to <code>'B'</code>, and a full byte is used to store it.</p>
<p>When referenced by a record number (that is, a key format of <code>'r'</code>), the <code>'t'</code> type will be stored in a fixed-length column-store, and will not have an out-of-band value to indicate the record does not exist. In this case, a 0 byte value is used to indicate the record does not exist. This means removing a record with Cursor.remove is equivalent to storing a value of 0 in the record with Cursor.update (and storing a value of 0 in the record will cause cursor scans to skip the record). Additionally, creating a record past the end of an object implies the creation of any missing intermediate records, with byte values of 0.</p>
<p>The <code>'u'</code> type is for raw byte arrays: if it appears at the end of a format string (including in the default <code>"u"</code> format for untyped tables), the size is not stored explicitly. When <code>'u'</code> appears within a format string, the size is stored as a 32-bit integer in the same byte order as the rest of the format string, followed by the data.</p>
<p>There is a default collator that gives lexicographic (byte-wise) comparisons, and the default encoding is designed so that lexicographic ordering of encoded keys is usually the expected ordering. For example, the variable-length encoding of integers is designed so that they have the natural integer ordering under the default collator.</p>
<p>See <a class="el" href="packing_lang_java.html">Packing and Unpacking Data in Java</a> for details of WiredTiger's packing format.</p>
<p>WiredTiger can also be extended with custom collators by implementing the <a class="el" href="struct_w_t___c_o_l_l_a_t_o_r.html" title="The interface implemented by applications to provide custom ordering of records. ">WT_COLLATOR</a> interface (C only).</p>
<h1><a class="anchor" id="schema_key_and_value_formats_lang_java"></a>
Key and value formats</h1>
<p>Every table has a key format and a value format as describe in <a class="el" href="schema_lang_java.html#schema_column_types_lang_java">Column types</a>. These types are configured when the table is created by passing <code>key_format</code> and <code>value_format</code> keys to Session.create.</p>
<p>For example, a simple row-store table with strings as both keys and values would be created as follows:</p>
<div class="fragment"><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"table:mytable"</span>, <span class="stringliteral">"key_format=S,value_format=S"</span>);</div></div><!-- fragment --><p> A simple column-store table with strings for values would be created as follows:</p>
<div class="fragment"><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"table:mytable"</span>, <span class="stringliteral">"key_format=r,value_format=S"</span>);</div></div><!-- fragment --> <h1><a class="anchor" id="schema_cursor_formats_lang_java"></a>
Cursor formats</h1>
<p>Cursors for a table have the same key format as the table itself. The key columns of a cursor are set with the <code>Cursor.putKey*</code> methods and accessed with the <code>Cursor.getKey*</code> methods. <code>Cursor.putKey*</code> methods must be called in the order the key columns are configured in <code>key_format</code>.</p>
<p>For example, setting the key for a row-store table with strings as keys would be done as follows:</p>
<div class="fragment"><div class="line"> <span class="comment">/* Set the cursor's string key. */</span></div><div class="line"> String key = <span class="stringliteral">"another key"</span>;</div><div class="line"> cursor.putKeyString(key);</div></div><!-- fragment --><p> For example, setting the key for a column-store table would be done as follows:</p>
<div class="fragment"><div class="line"> <span class="keywordtype">long</span> recno = 37; <span class="comment">/* Set the cursor's record number key. */</span></div><div class="line"> cursor.putKeyLong(recno);</div></div><!-- fragment --><p> A more complex example, setting a composite key for a row-store table where the key_format was <code>"SiH"</code>, would be done as follows:</p>
<div class="fragment"><div class="line"> <span class="comment">/* Set the cursor's "SiH" format composite key. */</span></div><div class="line"> cursor.putKeyString(<span class="stringliteral">"first"</span>);</div><div class="line"> cursor.putKeyInt(5);</div><div class="line"> cursor.putKeyShort((<span class="keywordtype">short</span>)7);</div></div><!-- fragment --><p> The key's values are accessed with successive calls to <code>Cursor.getKey*</code> methods:</p>
<div class="fragment"><div class="line"> String key; <span class="comment">/* Get the cursor's string key. */</span></div><div class="line"> key = cursor.getKeyString();</div></div><!-- fragment --><div class="fragment"><div class="line"> <span class="keywordtype">long</span> recno; <span class="comment">/* Get the cursor's record number key. */</span></div><div class="line"> recno = cursor.getKeyLong();</div></div><!-- fragment --><div class="fragment"><div class="line"> <span class="comment">/* Get the cursor's "SiH" format composite key. */</span></div><div class="line"> String first;</div><div class="line"> <span class="keywordtype">int</span> second;</div><div class="line"> <span class="keywordtype">short</span> third;</div><div class="line"></div><div class="line"> first = cursor.getKeyString();</div><div class="line"> second = cursor.getKeyInt();</div><div class="line"> third = cursor.getKeyShort();</div></div><!-- fragment --><p> Cursors for a table have the same value format as the table, unless a projection is configured with Session.open_cursor. See <a class="el" href="cursors_lang_java.html#cursor_projections_lang_java">Projections</a> for more information.</p>
<p><code>The</code> Cursor.putValue* methods are used to set value columns, and <code>Cursor.getValue*</code> are used to get value columns, in the same way as described for <code>Cursor.putKey*</code> and <code>Cursor.getKey*</code>.</p>
<h1><a class="anchor" id="schema_columns_lang_java"></a>
Columns</h1>
<p>The columns in a table can be assigned names by passing a <code>columns</code> key to Session.create. The column names are assigned first to the columns in the <code>key_format</code>, and then to the columns in <code>value_format</code>. There must be a name for every column, and no column names may be repeated.</p>
<p>For example, a column-store table with an employee ID as the key and three columns (department, salary and first year of employment), might be created as follows:</p>
<div class="fragment"><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Create a table with columns: keys are record numbers, values are</span></div><div class="line"><span class="comment"> * (string, signed 32-bit integer, unsigned 16-bit integer).</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"table:mytable"</span>,</div><div class="line"> <span class="stringliteral">"key_format=r,value_format=SiH,"</span> +</div><div class="line"> <span class="stringliteral">"columns=(id,department,salary,year-started)"</span>);</div></div><!-- fragment --><p> In this example, the key's column name is <code>id</code>, and the value's column names are <code>department</code>, <code>salary</code>, and <code>year-started</code> (where <code>id</code> maps to the column format <code>r</code>, <code>department</code> maps to the column value format <code>S</code>, <code>salary</code> maps to the value format <code>i</code> and <code>year-started</code> maps to the value format <code>H</code>).</p>
<p>Once the table is created, there is no need to call Session.create during subsequent runs of the application. However, it's worthwhile making the call anyway as it both verifies the table exists and the table schema matches the schema expected by the application.</p>
<h1><a class="anchor" id="schema_column_groups_lang_java"></a>
Column groups</h1>
<p>Once column names are assigned, they can be used to configure column groups. Column groups are primarily used to define storage in order to tune cache behavior, as each column group is stored in a separate file.</p>
<p>There are two steps involved in setting up column groups: first, pass a list of names for the column groups in the <code>colgroups</code> configuration key to Session.create. Then make a call to Session.create for each column group, using the URI <code>colgroup:<table>:<colgroup name></code> and a <code>columns</code> key in the configuration. Every column must appear in at least one column group; columns can be listed in multiple column groups, causing the column to be stored in multiple files.</p>
<p>For example, consider the following data being stored in a WiredTiger table:</p>
<div class="fragment"><div class="line"> <span class="comment">/* The class for the data we are storing in a WiredTiger table. */</span></div><div class="line"> <span class="keyword">static</span> <span class="keyword">class </span>PopRecord {</div><div class="line"> <span class="keyword">public</span> String country; <span class="comment">// Stored in database as fixed size char[5];</span></div><div class="line"> <span class="keyword">public</span> <span class="keywordtype">short</span> year;</div><div class="line"> <span class="keyword">public</span> <span class="keywordtype">long</span> population;</div><div class="line"> <span class="keyword">public</span> PopRecord(String country, <span class="keywordtype">short</span> year, <span class="keywordtype">long</span> population) {</div><div class="line"> this.country = country;</div><div class="line"> this.year = year;</div><div class="line"> this.population = population;</div><div class="line"> }</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="keyword">static</span> List<PopRecord> popData;</div><div class="line"></div><div class="line"> <span class="keyword">static</span> {</div><div class="line"> popData = <span class="keyword">new</span> ArrayList<PopRecord>();</div><div class="line"></div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"AU"</span>, (<span class="keywordtype">short</span>)1900, 4000000 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"AU"</span>, (<span class="keywordtype">short</span>)1950, 8267337 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"AU"</span>, (<span class="keywordtype">short</span>)2000, 19053186 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"CAN"</span>, (<span class="keywordtype">short</span>)1900, 5500000 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"CAN"</span>, (<span class="keywordtype">short</span>)1950, 14011422 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"CAN"</span>, (<span class="keywordtype">short</span>)2000, 31099561 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"UK"</span>, (<span class="keywordtype">short</span>)1900, 369000000 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"UK"</span>, (<span class="keywordtype">short</span>)1950, 50127000 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"UK"</span>, (<span class="keywordtype">short</span>)2000, 59522468 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"USA"</span>, (<span class="keywordtype">short</span>)1900, 76212168 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"USA"</span>, (<span class="keywordtype">short</span>)1950, 150697361 ));</div><div class="line"> popData.add(<span class="keyword">new</span> PopRecord(<span class="stringliteral">"USA"</span>, (<span class="keywordtype">short</span>)2000, 301279593 ));</div><div class="line"> };</div></div><!-- fragment --><p> If we primarily wanted to access the population information by itself, but still wanted population information included when accessing other information, we might store all of the columns in one file, and store an additional copy of the population column in another file:</p>
<div class="fragment"><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Create the population table.</span></div><div class="line"><span class="comment"> * Keys are record numbers, the format for values is (5-byte string,</span></div><div class="line"><span class="comment"> * long, long).</span></div><div class="line"><span class="comment"> * See ::wiredtiger_struct_pack for details of the format strings.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"table:poptable"</span>,</div><div class="line"> <span class="stringliteral">"key_format=r,value_format=5sHQ,"</span> +</div><div class="line"> <span class="stringliteral">"columns=(id,country,year,population),colgroups=(main,population)"</span>);</div><div class="line"></div><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Create two column groups: a primary column group with the country</span></div><div class="line"><span class="comment"> * code, year and population (named "main"), and a population column</span></div><div class="line"><span class="comment"> * group with the population by itself (named "population").</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"colgroup:poptable:main"</span>,</div><div class="line"> <span class="stringliteral">"columns=(country,year,population)"</span>);</div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"colgroup:poptable:population"</span>,</div><div class="line"> <span class="stringliteral">"columns=(population)"</span>);</div></div><!-- fragment --><p> Column groups always have the same key as the table. This is particularly useful for column stores, because record numbers are not stored explicitly on disk, so there is no repetition of keys across multiple files. Keys will be replicated in multiple files in the case of row-store column groups.</p>
<p>A cursor can be opened on a column group by passing the column group's URI to the Session.open_cursor method. For example, the population can be retrieved from both of the column groups we created:</p>
<div class="fragment"><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Open a cursor on the main column group, and return the information</span></div><div class="line"><span class="comment"> * for a particular country.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(<span class="stringliteral">"colgroup:poptable:main"</span>, null, null);</div><div class="line"> cursor.putKeyRecord(2);</div><div class="line"> <span class="keywordflow">if</span> ((ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#a7e25b2ced2cf3ec68bd5429bf921c79f">search</a>()) == 0) {</div><div class="line"> country = cursor.getValueString();</div><div class="line"> year = cursor.getValueShort();</div><div class="line"> population = cursor.getValueLong();</div><div class="line"> System.out.println(<span class="stringliteral">"ID 2: country "</span> + country +</div><div class="line"> <span class="stringliteral">", year "</span> + year + <span class="stringliteral">", population "</span> + population);</div><div class="line"> }</div></div><!-- fragment --><div class="fragment"><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Open a cursor on the population column group, and return the</span></div><div class="line"><span class="comment"> * population of a particular country.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(<span class="stringliteral">"colgroup:poptable:population"</span>, null, null);</div><div class="line"> cursor.putKeyRecord(2);</div><div class="line"> <span class="keywordflow">if</span> ((ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#a7e25b2ced2cf3ec68bd5429bf921c79f">search</a>()) == 0) {</div><div class="line"> population = cursor.getValueLong();</div><div class="line"> System.out.println(<span class="stringliteral">"ID 2: population "</span> + population);</div><div class="line"> }</div></div><!-- fragment --><p> Key columns may not be included in the list of columns for a column group. Because column groups always have the same key as the table, key columns for column groups are retrieved using Cursor.get_key, not Cursor.get_value.</p>
<h1><a class="anchor" id="schema_indices_lang_java"></a>
Indices</h1>
<p>Columns are also used to create and configure indices on tables.</p>
<p>Table indices are automatically updated whenever the table is modified.</p>
<p>Table index cursors are read-only and cannot be used for update operations.</p>
<p>To create a table index, call Session.create using the URI <code>index:<table>:<index name></code>, listing a column in the configuration.</p>
<p>Continuing the example, we might open an index on the <code>country</code> column:</p>
<div class="fragment"><div class="line"> <span class="comment">/* Create an index with a simple key. */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"index:poptable:country"</span>,</div><div class="line"> <span class="stringliteral">"columns=(country)"</span>);</div></div><!-- fragment --><p> Cursors are opened on indices by passing the index's URI to the Session.open_cursor method.</p>
<p>Index cursors use the specified index key columns for <code>Cursor.getKey*</code> and <code>Cursor.putKey*</code> calls. For example, we can retrieve information from the <code>country</code> index as follows:</p>
<div class="fragment"><div class="line"> <span class="comment">/* Search in a simple index. */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(<span class="stringliteral">"index:poptable:country"</span>, null, null);</div><div class="line"> cursor.putKeyString(<span class="stringliteral">"AU"</span>);</div><div class="line"> ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#a7e25b2ced2cf3ec68bd5429bf921c79f">search</a>();</div><div class="line"> country = cursor.getValueString();</div><div class="line"> year = cursor.getValueShort();</div><div class="line"> population = cursor.getValueLong();</div><div class="line"> System.out.println(<span class="stringliteral">"AU: country "</span> + country + <span class="stringliteral">", year "</span> + year +</div><div class="line"> <span class="stringliteral">", population "</span> + population);</div></div><!-- fragment --><p> To create an index with a composite key, specify more than one column to the Session.create call:</p>
<div class="fragment"><div class="line"> <span class="comment">/* Create an index with a composite key (country,year). */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"index:poptable:country_plus_year"</span>,</div><div class="line"> <span class="stringliteral">"columns=(country,year)"</span>);</div></div><!-- fragment --><p> To retrieve information from a composite index requires a more complicated set of <code>Cursor.putKey*</code> calls, but is otherwise the same:</p>
<div class="fragment"><div class="line"> <span class="comment">/* Search in a composite index. */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(</div><div class="line"> <span class="stringliteral">"index:poptable:country_plus_year"</span>, null, null);</div><div class="line"> cursor.putKeyString(<span class="stringliteral">"USA"</span>);</div><div class="line"> cursor.putKeyShort((<span class="keywordtype">short</span>)1900);</div><div class="line"> ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#a7e25b2ced2cf3ec68bd5429bf921c79f">search</a>();</div><div class="line"> country = cursor.getValueString();</div><div class="line"> year = cursor.getValueShort();</div><div class="line"> population = cursor.getValueLong();</div><div class="line"> System.out.println(<span class="stringliteral">"US 1900: country "</span> + country +</div><div class="line"> <span class="stringliteral">", year "</span> + year + <span class="stringliteral">", population "</span> + population);</div></div><!-- fragment --> <h1><a class="anchor" id="schema_index_immutable_lang_java"></a>
Immutable indices</h1>
<p>It is possible to create an index with the <code>immutable</code> configuration setting enabled. This setting tells WiredTiger that the index keys for a record do not change when records are updated. This is an optimization that it saves a remove and insert into the index whenever a value in the primary table is updated.</p>
<p>If immutable is configured when updates should alter the content of the index it is possible to corrupt data.</p>
<p>An example of using an immutable index is:</p>
<div class="fragment"><div class="line"> <span class="comment">/* Create an index with a simple key. */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"index:poptable:immutable_year"</span>,</div><div class="line"> <span class="stringliteral">"columns=(year),immutable"</span>);</div></div><!-- fragment --> <h1><a class="anchor" id="schema_index_projections_lang_java"></a>
Index cursor projections</h1>
<p>By default, index cursors return all of the table's value columns from <code>Cursor.getValue*</code> calls. The application can specify that a subset of the usual columns should be returned in calls to <code>Cursor.getValue</code> by appending a list of columns to the <code>uri</code> parameter of the Session.open_cursor call. This is called a <em>projection</em>, see <a class="el" href="cursors_lang_java.html#cursor_projections_lang_java">Projections</a> for more details.</p>
<p>In the case of index cursors, a projection can be used to avoid lookups in column groups that do not hold columns relevant to the operation.</p>
<p>The following example will return just the table's primary key (a record number, in this case) from the index:</p>
<div class="fragment"><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Use a projection to return just the table's record number key</span></div><div class="line"><span class="comment"> * from an index.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(<span class="stringliteral">"index:poptable:country_plus_year(id)"</span>, null, null);</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>()) == 0) {</div><div class="line"> country = cursor.getKeyString();</div><div class="line"> year = cursor.getKeyShort();</div><div class="line"> recno = cursor.getValueRecord();</div><div class="line"> System.out.println(<span class="stringliteral">"row ID "</span> + recno + <span class="stringliteral">": country "</span> + country +</div><div class="line"> <span class="stringliteral">", year "</span> + year);</div><div class="line"> }</div></div><!-- fragment --><p> Here is an example of a projection that returns a subset of columns from the index:</p>
<div class="fragment"><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Use a projection to return just the population column from an</span></div><div class="line"><span class="comment"> * index.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(</div><div class="line"> <span class="stringliteral">"index:poptable:country_plus_year(population)"</span>, null, null);</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>()) == 0) {</div><div class="line"> country = cursor.getKeyString();</div><div class="line"> year = cursor.getKeyShort();</div><div class="line"> population = cursor.getValueLong();</div><div class="line"> System.out.println(<span class="stringliteral">"population "</span> + population +</div><div class="line"> <span class="stringliteral">": country "</span> + country + <span class="stringliteral">", year "</span> + year);</div><div class="line"> }</div></div><!-- fragment --><p> For performance reasons, it may be desirable to include all columns for a performance-critical operation in an index, so that it is possible to perform index-only lookups where no column group from the table is accessed. In this case, all of the "hot" columns should be included in the index (always list the "real" index key columns first, so they will determine the sort order). Then, open a cursor on the index that doesn't return any value columns, and no column group will be accessed.</p>
<div class="fragment"><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Use a projection to avoid accessing any other column groups when</span></div><div class="line"><span class="comment"> * using an index: supply an empty list of value columns.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(</div><div class="line"> <span class="stringliteral">"index:poptable:country_plus_year()"</span>, null, null);</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>()) == 0) {</div><div class="line"> country = cursor.getKeyString();</div><div class="line"> year = cursor.getKeyShort();</div><div class="line"> System.out.println(<span class="stringliteral">"country "</span> + country + <span class="stringliteral">", year "</span> + year);</div><div class="line"> }</div></div><!-- fragment --><p> Index cursors for column-store objects may not be created using the record number as the index key (there is no use for a secondary index on a column-store where the index key is the record number).</p>
<h1><a class="anchor" id="schema_examples_lang_java"></a>
Code samples</h1>
<p>The code included above was taken from the complete example program ex_schema.java.</p>
<p>Here is another example program, ex_call_center.java.</p>
<div class="fragment"><div class="line"><span class="comment">/*</span></div><div class="line"><span class="comment"> * In SQL, the tables are described as follows:</span></div><div class="line"><span class="comment"> *</span></div><div class="line"><span class="comment"> * CREATE TABLE Customers(id INTEGER PRIMARY KEY,</span></div><div class="line"><span class="comment"> * name VARCHAR(30), address VARCHAR(50), phone VARCHAR(15))</span></div><div class="line"><span class="comment"> * CREATE INDEX CustomersPhone ON Customers(phone)</span></div><div class="line"><span class="comment"> *</span></div><div class="line"><span class="comment"> * CREATE TABLE Calls(id INTEGER PRIMARY KEY, call_date DATE,</span></div><div class="line"><span class="comment"> * cust_id INTEGER, emp_id INTEGER, call_type VARCHAR(12),</span></div><div class="line"><span class="comment"> * notes VARCHAR(25))</span></div><div class="line"><span class="comment"> * CREATE INDEX CallsCustDate ON Calls(cust_id, call_date)</span></div><div class="line"><span class="comment"> *</span></div><div class="line"><span class="comment"> * In this example, both tables will use record numbers for their IDs, which</span></div><div class="line"><span class="comment"> * will be the key. The C structs for the records are as follows.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"></div><div class="line"><span class="comment">/* Customer records. */</span></div><div class="line"><span class="keyword">class </span>Customer {</div><div class="line"> <span class="keyword">public</span> <span class="keywordtype">long</span> id;</div><div class="line"> <span class="keyword">public</span> String name;</div><div class="line"> <span class="keyword">public</span> String address;</div><div class="line"> <span class="keyword">public</span> String phone;</div><div class="line"> <span class="keyword">public</span> Customer(<span class="keywordtype">long</span> <span class="keywordtype">id</span>, String name, String address, String phone) {</div><div class="line"> this.<span class="keywordtype">id</span> = id;</div><div class="line"> this.name = name;</div><div class="line"> this.address = address;</div><div class="line"> this.phone = phone;</div><div class="line"> }</div><div class="line"> <span class="keyword">public</span> Customer() {}</div><div class="line">}</div><div class="line"></div><div class="line"><span class="comment">/* Call records. */</span></div><div class="line"><span class="keyword">class </span>Call {</div><div class="line"> <span class="keyword">public</span> <span class="keywordtype">long</span> id;</div><div class="line"> <span class="keyword">public</span> <span class="keywordtype">long</span> call_date;</div><div class="line"> <span class="keyword">public</span> <span class="keywordtype">long</span> cust_id;</div><div class="line"> <span class="keyword">public</span> <span class="keywordtype">long</span> emp_id;</div><div class="line"> <span class="keyword">public</span> String call_type;</div><div class="line"> <span class="keyword">public</span> String notes;</div><div class="line"> <span class="keyword">public</span> Call(<span class="keywordtype">long</span> <span class="keywordtype">id</span>, <span class="keywordtype">long</span> call_date, <span class="keywordtype">long</span> cust_id, <span class="keywordtype">long</span> emp_id,</div><div class="line"> String call_type, String notes) {</div><div class="line"> this.<span class="keywordtype">id</span> = id;</div><div class="line"> this.call_date = call_date;</div><div class="line"> this.cust_id = cust_id;</div><div class="line"> this.emp_id = emp_id;</div><div class="line"> this.call_type = call_type;</div><div class="line"> this.notes = notes;</div><div class="line"> }</div><div class="line"> <span class="keyword">public</span> Call() {}</div><div class="line">}</div></div><!-- fragment --><div class="fragment"><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Create the customers table, give names and types to the columns.</span></div><div class="line"><span class="comment"> * The columns will be stored in two groups: "main" and "address",</span></div><div class="line"><span class="comment"> * created below.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"table:customers"</span>,</div><div class="line"> <span class="stringliteral">"key_format=r,"</span> +</div><div class="line"> <span class="stringliteral">"value_format=SSS,"</span> +</div><div class="line"> <span class="stringliteral">"columns=(id,name,address,phone),"</span> +</div><div class="line"> <span class="stringliteral">"colgroups=(main,address)"</span>);</div><div class="line"></div><div class="line"> <span class="comment">/* Create the main column group with value columns except address. */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(</div><div class="line"> <span class="stringliteral">"colgroup:customers:main"</span>, <span class="stringliteral">"columns=(name,phone)"</span>);</div><div class="line"></div><div class="line"> <span class="comment">/* Create the address column group with just the address. */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(</div><div class="line"> <span class="stringliteral">"colgroup:customers:address"</span>, <span class="stringliteral">"columns=(address)"</span>);</div><div class="line"></div><div class="line"> <span class="comment">/* Create an index on the customer table by phone number. */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(</div><div class="line"> <span class="stringliteral">"index:customers:phone"</span>, <span class="stringliteral">"columns=(phone)"</span>);</div><div class="line"></div><div class="line"> <span class="comment">/* Populate the customers table with some data. */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(<span class="stringliteral">"table:customers"</span>, null, <span class="stringliteral">"append"</span>);</div><div class="line"> <span class="keywordflow">for</span> (Customer cust : custSample) {</div><div class="line"> cursor.putValueString(cust.name);</div><div class="line"> cursor.putValueString(cust.address);</div><div class="line"> cursor.putValueString(cust.phone);</div><div class="line"> ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3">insert</a>();</div><div class="line"> }</div><div class="line"> ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#aeea071f192cab12245a50fbe71c3460b">close</a>();</div><div class="line"></div><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Create the calls table, give names and types to the columns. All the</span></div><div class="line"><span class="comment"> * columns will be stored together, so no column groups are declared.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"table:calls"</span>,</div><div class="line"> <span class="stringliteral">"key_format=r,"</span> +</div><div class="line"> <span class="stringliteral">"value_format=qrrSS,"</span> +</div><div class="line"> <span class="stringliteral">"columns=(id,call_date,cust_id,emp_id,call_type,notes)"</span>);</div><div class="line"></div><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Create an index on the calls table with a composite key of cust_id</span></div><div class="line"><span class="comment"> * and call_date.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> ret = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(<span class="stringliteral">"index:calls:cust_date"</span>,</div><div class="line"> <span class="stringliteral">"columns=(cust_id,call_date)"</span>);</div><div class="line"></div><div class="line"> <span class="comment">/* Populate the calls table with some data. */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(<span class="stringliteral">"table:calls"</span>, null, <span class="stringliteral">"append"</span>);</div><div class="line"> <span class="keywordflow">for</span> (Call call : callSample) {</div><div class="line"> cursor.putValueLong(call.call_date);</div><div class="line"> cursor.putValueRecord(call.cust_id);</div><div class="line"> cursor.putValueRecord(call.emp_id);</div><div class="line"> cursor.putValueString(call.call_type);</div><div class="line"> cursor.putValueString(call.notes);</div><div class="line"> ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3">insert</a>();</div><div class="line"> }</div><div class="line"> ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#aeea071f192cab12245a50fbe71c3460b">close</a>();</div><div class="line"></div><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * First query: a call arrives. In SQL:</span></div><div class="line"><span class="comment"> *</span></div><div class="line"><span class="comment"> * SELECT id, name FROM Customers WHERE phone=?</span></div><div class="line"><span class="comment"> *</span></div><div class="line"><span class="comment"> * Use the cust_phone index, lookup by phone number to fill the</span></div><div class="line"><span class="comment"> * customer record. The cursor will have a key format of "S" for a</span></div><div class="line"><span class="comment"> * string because the cust_phone index has a single column ("phone"),</span></div><div class="line"><span class="comment"> * which is of type "S".</span></div><div class="line"><span class="comment"> *</span></div><div class="line"><span class="comment"> * Specify the columns we want: the customer ID and the name. This</span></div><div class="line"><span class="comment"> * means the cursor's value format will be "rS".</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(</div><div class="line"> <span class="stringliteral">"index:customers:phone(id,name)"</span>, null, null);</div><div class="line"> cursor.putKeyString(<span class="stringliteral">"123-456-7890"</span>);</div><div class="line"> ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#a7e25b2ced2cf3ec68bd5429bf921c79f">search</a>();</div><div class="line"> <span class="keywordflow">if</span> (ret == 0) {</div><div class="line"> Customer cust = <span class="keyword">new</span> Customer();</div><div class="line"> cust.id = cursor.getValueRecord();</div><div class="line"> cust.name = cursor.getValueString();</div><div class="line"> System.out.println(<span class="stringliteral">"Read customer record for "</span> + cust.name +</div><div class="line"> <span class="stringliteral">" (ID "</span> + cust.id + <span class="stringliteral">")"</span>);</div><div class="line"> }</div><div class="line"> ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#aeea071f192cab12245a50fbe71c3460b">close</a>();</div><div class="line"></div><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Next query: get the recent order history. In SQL:</span></div><div class="line"><span class="comment"> *</span></div><div class="line"><span class="comment"> * SELECT * FROM Calls WHERE cust_id=? ORDER BY call_date DESC LIMIT 3</span></div><div class="line"><span class="comment"> *</span></div><div class="line"><span class="comment"> * Use the call_cust_date index to find the matching calls. Since it is</span></div><div class="line"><span class="comment"> * is in increasing order by date for a given customer, we want to start</span></div><div class="line"><span class="comment"> * with the last record for the customer and work backwards.</span></div><div class="line"><span class="comment"> *</span></div><div class="line"><span class="comment"> * Specify a subset of columns to be returned. (Note that if these were</span></div><div class="line"><span class="comment"> * all covered by the index, the primary would not have to be accessed.)</span></div><div class="line"><span class="comment"> * Stop after getting 3 records.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> cursor = session.<a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(</div><div class="line"> <span class="stringliteral">"index:calls:cust_date(cust_id,call_type,notes)"</span>,</div><div class="line"> null, null);</div><div class="line"></div><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * The keys in the index are (cust_id,call_date) -- we want the largest</span></div><div class="line"><span class="comment"> * call date for a given cust_id. Search for (cust_id+1,0), then work</span></div><div class="line"><span class="comment"> * backwards.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> <span class="keywordtype">long</span> custid = 1;</div><div class="line"> cursor.putKeyRecord(custid + 1);</div><div class="line"> cursor.putKeyLong(0);</div><div class="line"> nearstatus = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#a8068ddce20d0775f26f6dac6e5eb209c">search_near</a>();</div><div class="line"></div><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * If the table is empty, search_near will return WT_NOTFOUND, else the</span></div><div class="line"><span class="comment"> * cursor will be positioned on a matching key if one exists, or an</span></div><div class="line"><span class="comment"> * adjacent key if one does not. If the positioned key is equal to or</span></div><div class="line"><span class="comment"> * larger than the search key, go back one.</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> <span class="keywordflow">if</span> (ret == 0 && (nearstatus == SearchStatus.LARGER ||</div><div class="line"> nearstatus == SearchStatus.FOUND))</div><div class="line"> ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#a43d6664d2f68902aa63f933864242e76">prev</a>();</div><div class="line"> <span class="keywordflow">for</span> (count = 0; ret == 0 && count < 3; ++count) {</div><div class="line"> Call call = <span class="keyword">new</span> Call();</div><div class="line"> call.cust_id = cursor.getValueRecord();</div><div class="line"> call.call_type = cursor.getValueString();</div><div class="line"> call.notes = cursor.getValueString();</div><div class="line"> <span class="keywordflow">if</span> (call.cust_id != custid)</div><div class="line"> <span class="keywordflow">break</span>;</div><div class="line"> System.out.println(<span class="stringliteral">"Call record: customer "</span> + call.cust_id +</div><div class="line"> <span class="stringliteral">" ("</span> + call.call_type +</div><div class="line"> <span class="stringliteral">": "</span> + call.notes + <span class="stringliteral">")"</span>);</div><div class="line"> ret = cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#a43d6664d2f68902aa63f933864242e76">prev</a>();</div><div class="line"> }</div></div><!-- fragment --></div></div><!-- contents -->
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