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| |
| <h1 id="virtual-columns">Virtual Columns</h1> |
| |
| <p>Virtual columns are queryable column "views" created from a set of columns during a query. </p> |
| |
| <p>A virtual column can potentially draw from multiple underlying columns, although a virtual column always presents itself as a single column.</p> |
| |
| <p>Virtual columns can be used as dimensions or as inputs to aggregators.</p> |
| |
| <p>Each Druid query can accept a list of virtual columns as a parameter. The following scan query is provided as an example:</p> |
| <div class="highlight"><pre><code class="language-text" data-lang="text"><span></span>{ |
| "queryType": "scan", |
| "dataSource": "page_data", |
| "columns":[], |
| "virtualColumns": [ |
| { |
| "type": "expression", |
| "name": "fooPage", |
| "expression": "concat('foo' + page)", |
| "outputType": "STRING" |
| }, |
| { |
| "type": "expression", |
| "name": "tripleWordCount", |
| "expression": "wordCount * 3", |
| "outputType": "LONG" |
| } |
| ], |
| "intervals": [ |
| "2013-01-01/2019-01-02" |
| ] |
| } |
| </code></pre></div> |
| <h2 id="virtual-column-types">Virtual Column Types</h2> |
| |
| <h3 id="expression-virtual-column">Expression virtual column</h3> |
| |
| <p>The expression virtual column has the following syntax:</p> |
| <div class="highlight"><pre><code class="language-text" data-lang="text"><span></span>{ |
| "type": "expression", |
| "name": <name of the virtual column>, |
| "expression": <row expression>, |
| "outputType": <output value type of expression> |
| } |
| </code></pre></div> |
| <table><thead> |
| <tr> |
| <th>property</th> |
| <th>description</th> |
| <th>required?</th> |
| </tr> |
| </thead><tbody> |
| <tr> |
| <td>name</td> |
| <td>The name of the virtual column.</td> |
| <td>yes</td> |
| </tr> |
| <tr> |
| <td>expression</td> |
| <td>An <a href="../misc/math-expr.html">expression</a> that takes a row as input and outputs a value for the virtual column.</td> |
| <td>yes</td> |
| </tr> |
| <tr> |
| <td>outputType</td> |
| <td>The expression's output will be coerced to this type. Can be LONG, FLOAT, DOUBLE, or STRING.</td> |
| <td>no, default is FLOAT</td> |
| </tr> |
| </tbody></table> |
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