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| |
| <h1 id="scan-query">Scan query</h1> |
| |
| <p>The Scan query returns raw Apache Druid (incubating) rows in streaming mode. The biggest difference between the Select query and the Scan |
| query is that the Scan query does not retain all the returned rows in memory before they are returned to the client.<br> |
| The Select query <em>will</em> retain the rows in memory, causing memory pressure if too many rows are returned.<br> |
| The Scan query can return all the rows without issuing another pagination query.</p> |
| |
| <p>In addition to straightforward usage where a Scan query is issued to the Broker, the Scan query can also be issued |
| directly to Historical processes or streaming ingestion tasks. This can be useful if you want to retrieve large |
| amounts of data in parallel.</p> |
| |
| <p>An example Scan query object is shown below:</p> |
| <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span> <span class="p">{</span> |
| <span class="nt">"queryType"</span><span class="p">:</span> <span class="s2">"scan"</span><span class="p">,</span> |
| <span class="nt">"dataSource"</span><span class="p">:</span> <span class="s2">"wikipedia"</span><span class="p">,</span> |
| <span class="nt">"resultFormat"</span><span class="p">:</span> <span class="s2">"list"</span><span class="p">,</span> |
| <span class="nt">"columns"</span><span class="p">:[],</span> |
| <span class="nt">"intervals"</span><span class="p">:</span> <span class="p">[</span> |
| <span class="s2">"2013-01-01/2013-01-02"</span> |
| <span class="p">],</span> |
| <span class="nt">"batchSize"</span><span class="p">:</span><span class="mi">20480</span><span class="p">,</span> |
| <span class="nt">"limit"</span><span class="p">:</span><span class="mi">3</span> |
| <span class="p">}</span> |
| </code></pre></div> |
| <p>The following are the main parameters for Scan queries:</p> |
| |
| <table><thead> |
| <tr> |
| <th>property</th> |
| <th>description</th> |
| <th>required?</th> |
| </tr> |
| </thead><tbody> |
| <tr> |
| <td>queryType</td> |
| <td>This String should always be "scan"; this is the first thing Druid looks at to figure out how to interpret the query</td> |
| <td>yes</td> |
| </tr> |
| <tr> |
| <td>dataSource</td> |
| <td>A String or Object defining the data source to query, very similar to a table in a relational database. See <a href="../querying/datasource.html">DataSource</a> for more information.</td> |
| <td>yes</td> |
| </tr> |
| <tr> |
| <td>intervals</td> |
| <td>A JSON Object representing ISO-8601 Intervals. This defines the time ranges to run the query over.</td> |
| <td>yes</td> |
| </tr> |
| <tr> |
| <td>resultFormat</td> |
| <td>How the results are represented: list, compactedList or valueVector. Currently only <code>list</code> and <code>compactedList</code> are supported. Default is <code>list</code></td> |
| <td>no</td> |
| </tr> |
| <tr> |
| <td>filter</td> |
| <td>See <a href="../querying/filters.html">Filters</a></td> |
| <td>no</td> |
| </tr> |
| <tr> |
| <td>columns</td> |
| <td>A String array of dimensions and metrics to scan. If left empty, all dimensions and metrics are returned.</td> |
| <td>no</td> |
| </tr> |
| <tr> |
| <td>batchSize</td> |
| <td>How many rows buffered before return to client. Default is <code>20480</code></td> |
| <td>no</td> |
| </tr> |
| <tr> |
| <td>limit</td> |
| <td>How many rows to return. If not specified, all rows will be returned.</td> |
| <td>no</td> |
| </tr> |
| <tr> |
| <td>order</td> |
| <td>The ordering of returned rows based on timestamp. "ascending", "descending", and "none" (default) are supported. Currently, "ascending" and "descending" are only supported for queries where the <code>__time</code> column is included in the <code>columns</code> field and the requirements outlined in the <a href="#time-ordering">time ordering</a> section are met.</td> |
| <td>none</td> |
| </tr> |
| <tr> |
| <td>legacy</td> |
| <td>Return results consistent with the legacy "scan-query" contrib extension. Defaults to the value set by <code>druid.query.scan.legacy</code>, which in turn defaults to false. See <a href="#legacy-mode">Legacy mode</a> for details.</td> |
| <td>no</td> |
| </tr> |
| <tr> |
| <td>context</td> |
| <td>An additional JSON Object which can be used to specify certain flags (see the Query Context Properties section below).</td> |
| <td>no</td> |
| </tr> |
| </tbody></table> |
| |
| <h2 id="example-results">Example results</h2> |
| |
| <p>The format of the result when resultFormat equals <code>list</code>:</p> |
| <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span> <span class="p">[{</span> |
| <span class="nt">"segmentId"</span> <span class="p">:</span> <span class="s2">"wikipedia_editstream_2012-12-29T00:00:00.000Z_2013-01-10T08:00:00.000Z_2013-01-10T08:13:47.830Z_v9"</span><span class="p">,</span> |
| <span class="nt">"columns"</span> <span class="p">:</span> <span class="p">[</span> |
| <span class="s2">"timestamp"</span><span class="p">,</span> |
| <span class="s2">"robot"</span><span class="p">,</span> |
| <span class="s2">"namespace"</span><span class="p">,</span> |
| <span class="s2">"anonymous"</span><span class="p">,</span> |
| <span class="s2">"unpatrolled"</span><span class="p">,</span> |
| <span class="s2">"page"</span><span class="p">,</span> |
| <span class="s2">"language"</span><span class="p">,</span> |
| <span class="s2">"newpage"</span><span class="p">,</span> |
| <span class="s2">"user"</span><span class="p">,</span> |
| <span class="s2">"count"</span><span class="p">,</span> |
| <span class="s2">"added"</span><span class="p">,</span> |
| <span class="s2">"delta"</span><span class="p">,</span> |
| <span class="s2">"variation"</span><span class="p">,</span> |
| <span class="s2">"deleted"</span> |
| <span class="p">],</span> |
| <span class="nt">"events"</span> <span class="p">:</span> <span class="p">[</span> <span class="p">{</span> |
| <span class="nt">"timestamp"</span> <span class="p">:</span> <span class="s2">"2013-01-01T00:00:00.000Z"</span><span class="p">,</span> |
| <span class="nt">"robot"</span> <span class="p">:</span> <span class="s2">"1"</span><span class="p">,</span> |
| <span class="nt">"namespace"</span> <span class="p">:</span> <span class="s2">"article"</span><span class="p">,</span> |
| <span class="nt">"anonymous"</span> <span class="p">:</span> <span class="s2">"0"</span><span class="p">,</span> |
| <span class="nt">"unpatrolled"</span> <span class="p">:</span> <span class="s2">"0"</span><span class="p">,</span> |
| <span class="nt">"page"</span> <span class="p">:</span> <span class="s2">"11._korpus_(NOVJ)"</span><span class="p">,</span> |
| <span class="nt">"language"</span> <span class="p">:</span> <span class="s2">"sl"</span><span class="p">,</span> |
| <span class="nt">"newpage"</span> <span class="p">:</span> <span class="s2">"0"</span><span class="p">,</span> |
| <span class="nt">"user"</span> <span class="p">:</span> <span class="s2">"EmausBot"</span><span class="p">,</span> |
| <span class="nt">"count"</span> <span class="p">:</span> <span class="mf">1.0</span><span class="p">,</span> |
| <span class="nt">"added"</span> <span class="p">:</span> <span class="mf">39.0</span><span class="p">,</span> |
| <span class="nt">"delta"</span> <span class="p">:</span> <span class="mf">39.0</span><span class="p">,</span> |
| <span class="nt">"variation"</span> <span class="p">:</span> <span class="mf">39.0</span><span class="p">,</span> |
| <span class="nt">"deleted"</span> <span class="p">:</span> <span class="mf">0.0</span> |
| <span class="p">},</span> <span class="p">{</span> |
| <span class="nt">"timestamp"</span> <span class="p">:</span> <span class="s2">"2013-01-01T00:00:00.000Z"</span><span class="p">,</span> |
| <span class="nt">"robot"</span> <span class="p">:</span> <span class="s2">"0"</span><span class="p">,</span> |
| <span class="nt">"namespace"</span> <span class="p">:</span> <span class="s2">"article"</span><span class="p">,</span> |
| <span class="nt">"anonymous"</span> <span class="p">:</span> <span class="s2">"0"</span><span class="p">,</span> |
| <span class="nt">"unpatrolled"</span> <span class="p">:</span> <span class="s2">"0"</span><span class="p">,</span> |
| <span class="nt">"page"</span> <span class="p">:</span> <span class="s2">"112_U.S._580"</span><span class="p">,</span> |
| <span class="nt">"language"</span> <span class="p">:</span> <span class="s2">"en"</span><span class="p">,</span> |
| <span class="nt">"newpage"</span> <span class="p">:</span> <span class="s2">"1"</span><span class="p">,</span> |
| <span class="nt">"user"</span> <span class="p">:</span> <span class="s2">"MZMcBride"</span><span class="p">,</span> |
| <span class="nt">"count"</span> <span class="p">:</span> <span class="mf">1.0</span><span class="p">,</span> |
| <span class="nt">"added"</span> <span class="p">:</span> <span class="mf">70.0</span><span class="p">,</span> |
| <span class="nt">"delta"</span> <span class="p">:</span> <span class="mf">70.0</span><span class="p">,</span> |
| <span class="nt">"variation"</span> <span class="p">:</span> <span class="mf">70.0</span><span class="p">,</span> |
| <span class="nt">"deleted"</span> <span class="p">:</span> <span class="mf">0.0</span> |
| <span class="p">},</span> <span class="p">{</span> |
| <span class="nt">"timestamp"</span> <span class="p">:</span> <span class="s2">"2013-01-01T00:00:00.000Z"</span><span class="p">,</span> |
| <span class="nt">"robot"</span> <span class="p">:</span> <span class="s2">"0"</span><span class="p">,</span> |
| <span class="nt">"namespace"</span> <span class="p">:</span> <span class="s2">"article"</span><span class="p">,</span> |
| <span class="nt">"anonymous"</span> <span class="p">:</span> <span class="s2">"0"</span><span class="p">,</span> |
| <span class="nt">"unpatrolled"</span> <span class="p">:</span> <span class="s2">"0"</span><span class="p">,</span> |
| <span class="nt">"page"</span> <span class="p">:</span> <span class="s2">"113_U.S._243"</span><span class="p">,</span> |
| <span class="nt">"language"</span> <span class="p">:</span> <span class="s2">"en"</span><span class="p">,</span> |
| <span class="nt">"newpage"</span> <span class="p">:</span> <span class="s2">"1"</span><span class="p">,</span> |
| <span class="nt">"user"</span> <span class="p">:</span> <span class="s2">"MZMcBride"</span><span class="p">,</span> |
| <span class="nt">"count"</span> <span class="p">:</span> <span class="mf">1.0</span><span class="p">,</span> |
| <span class="nt">"added"</span> <span class="p">:</span> <span class="mf">77.0</span><span class="p">,</span> |
| <span class="nt">"delta"</span> <span class="p">:</span> <span class="mf">77.0</span><span class="p">,</span> |
| <span class="nt">"variation"</span> <span class="p">:</span> <span class="mf">77.0</span><span class="p">,</span> |
| <span class="nt">"deleted"</span> <span class="p">:</span> <span class="mf">0.0</span> |
| <span class="p">}</span> <span class="p">]</span> |
| <span class="p">}</span> <span class="p">]</span> |
| </code></pre></div> |
| <p>The format of the result when resultFormat equals <code>compactedList</code>:</p> |
| <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span> <span class="p">[{</span> |
| <span class="nt">"segmentId"</span> <span class="p">:</span> <span class="s2">"wikipedia_editstream_2012-12-29T00:00:00.000Z_2013-01-10T08:00:00.000Z_2013-01-10T08:13:47.830Z_v9"</span><span class="p">,</span> |
| <span class="nt">"columns"</span> <span class="p">:</span> <span class="p">[</span> |
| <span class="s2">"timestamp"</span><span class="p">,</span> <span class="s2">"robot"</span><span class="p">,</span> <span class="s2">"namespace"</span><span class="p">,</span> <span class="s2">"anonymous"</span><span class="p">,</span> <span class="s2">"unpatrolled"</span><span class="p">,</span> <span class="s2">"page"</span><span class="p">,</span> <span class="s2">"language"</span><span class="p">,</span> <span class="s2">"newpage"</span><span class="p">,</span> <span class="s2">"user"</span><span class="p">,</span> <span class="s2">"count"</span><span class="p">,</span> <span class="s2">"added"</span><span class="p">,</span> <span class="s2">"delta"</span><span class="p">,</span> <span class="s2">"variation"</span><span class="p">,</span> <span class="s2">"deleted"</span> |
| <span class="p">],</span> |
| <span class="nt">"events"</span> <span class="p">:</span> <span class="p">[</span> |
| <span class="p">[</span><span class="s2">"2013-01-01T00:00:00.000Z"</span><span class="p">,</span> <span class="s2">"1"</span><span class="p">,</span> <span class="s2">"article"</span><span class="p">,</span> <span class="s2">"0"</span><span class="p">,</span> <span class="s2">"0"</span><span class="p">,</span> <span class="s2">"11._korpus_(NOVJ)"</span><span class="p">,</span> <span class="s2">"sl"</span><span class="p">,</span> <span class="s2">"0"</span><span class="p">,</span> <span class="s2">"EmausBot"</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">,</span> <span class="mf">39.0</span><span class="p">,</span> <span class="mf">39.0</span><span class="p">,</span> <span class="mf">39.0</span><span class="p">,</span> <span class="mf">0.0</span><span class="p">],</span> |
| <span class="p">[</span><span class="s2">"2013-01-01T00:00:00.000Z"</span><span class="p">,</span> <span class="s2">"0"</span><span class="p">,</span> <span class="s2">"article"</span><span class="p">,</span> <span class="s2">"0"</span><span class="p">,</span> <span class="s2">"0"</span><span class="p">,</span> <span class="s2">"112_U.S._580"</span><span class="p">,</span> <span class="s2">"en"</span><span class="p">,</span> <span class="s2">"1"</span><span class="p">,</span> <span class="s2">"MZMcBride"</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">,</span> <span class="mf">70.0</span><span class="p">,</span> <span class="mf">70.0</span><span class="p">,</span> <span class="mf">70.0</span><span class="p">,</span> <span class="mf">0.0</span><span class="p">],</span> |
| <span class="p">[</span><span class="s2">"2013-01-01T00:00:00.000Z"</span><span class="p">,</span> <span class="s2">"0"</span><span class="p">,</span> <span class="s2">"article"</span><span class="p">,</span> <span class="s2">"0"</span><span class="p">,</span> <span class="s2">"0"</span><span class="p">,</span> <span class="s2">"113_U.S._243"</span><span class="p">,</span> <span class="s2">"en"</span><span class="p">,</span> <span class="s2">"1"</span><span class="p">,</span> <span class="s2">"MZMcBride"</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">,</span> <span class="mf">77.0</span><span class="p">,</span> <span class="mf">77.0</span><span class="p">,</span> <span class="mf">77.0</span><span class="p">,</span> <span class="mf">0.0</span><span class="p">]</span> |
| <span class="p">]</span> |
| <span class="p">}</span> <span class="p">]</span> |
| </code></pre></div> |
| <h2 id="time-ordering">Time Ordering</h2> |
| |
| <p>The Scan query currently supports ordering based on timestamp for non-legacy queries. Note that using time ordering |
| will yield results that do not indicate which segment rows are from (<code>segmentId</code> will show up as <code>null</code>). Furthermore, |
| time ordering is only supported where the result set limit is less than <code>druid.query.scan.maxRowsQueuedForOrdering</code> |
| rows <strong>or</strong> all segments scanned have fewer than <code>druid.query.scan.maxSegmentPartitionsOrderedInMemory</code> partitions. Also, |
| time ordering is not supported for queries issued directly to historicals unless a list of segments is specified. The |
| reasoning behind these limitations is that the implementation of time ordering uses two strategies that can consume too |
| much heap memory if left unbounded. These strategies (listed below) are chosen on a per-Historical basis depending on |
| query result set limit and the number of segments being scanned.</p> |
| |
| <ol> |
| <li><p>Priority Queue: Each segment on a Historical is opened sequentially. Every row is added to a bounded priority |
| queue which is ordered by timestamp. For every row above the result set limit, the row with the earliest (if descending) |
| or latest (if ascending) timestamp will be dequeued. After every row has been processed, the sorted contents of the |
| priority queue are streamed back to the Broker(s) in batches. Attempting to load too many rows into memory runs the |
| risk of Historical nodes running out of memory. The <code>druid.query.scan.maxRowsQueuedForOrdering</code> property protects |
| from this by limiting the number of rows in the query result set when time ordering is used.</p></li> |
| <li><p>N-Way Merge: For each segment, each partition is opened in parallel. Since each partition's rows are already |
| time-ordered, an n-way merge can be performed on the results from each partition. This approach doesn't persist the entire |
| result set in memory (like the Priority Queue) as it streams back batches as they are returned from the merge function. |
| However, attempting to query too many partition could also result in high memory usage due to the need to open |
| decompression and decoding buffers for each. The <code>druid.query.scan.maxSegmentPartitionsOrderedInMemory</code> limit protects |
| from this by capping the number of partitions opened at any times when time ordering is used.</p></li> |
| </ol> |
| |
| <p>Both <code>druid.query.scan.maxRowsQueuedForOrdering</code> and <code>druid.query.scan.maxSegmentPartitionsOrderedInMemory</code> are |
| configurable and can be tuned based on hardware specs and number of dimensions being queried. These config properties |
| can also be overridden using the <code>maxRowsQueuedForOrdering</code> and <code>maxSegmentPartitionsOrderedInMemory</code> properties in |
| the query context (see the Query Context Properties section).</p> |
| |
| <h2 id="legacy-mode">Legacy mode</h2> |
| |
| <p>The Scan query supports a legacy mode designed for protocol compatibility with the former scan-query contrib extension. |
| In legacy mode you can expect the following behavior changes:</p> |
| |
| <ul> |
| <li>The <strong>time column is returned as "timestamp" rather than "</strong>time". This will take precedence over any other column |
| you may have that is named "timestamp".</li> |
| <li>The __time column is included in the list of columns even if you do not specifically ask for it.</li> |
| <li>Timestamps are returned as ISO8601 time strings rather than integers (milliseconds since 1970-01-01 00:00:00 UTC).</li> |
| </ul> |
| |
| <p>Legacy mode can be triggered either by passing <code>"legacy" : true</code> in your query JSON, or by setting |
| <code>druid.query.scan.legacy = true</code> on your Druid processes. If you were previously using the scan-query contrib extension, |
| the best way to migrate is to activate legacy mode during a rolling upgrade, then switch it off after the upgrade |
| is complete.</p> |
| |
| <h2 id="configuration-properties">Configuration Properties</h2> |
| |
| <p>Configuration properties:</p> |
| |
| <table><thead> |
| <tr> |
| <th>property</th> |
| <th>description</th> |
| <th>values</th> |
| <th>default</th> |
| </tr> |
| </thead><tbody> |
| <tr> |
| <td>druid.query.scan.maxRowsQueuedForOrdering</td> |
| <td>The maximum number of rows returned when time ordering is used</td> |
| <td>An integer in [1, 2147483647]</td> |
| <td>100000</td> |
| </tr> |
| <tr> |
| <td>druid.query.scan.maxSegmentPartitionsOrderedInMemory</td> |
| <td>The maximum number of segments scanned per historical when time ordering is used</td> |
| <td>An integer in [1, 2147483647]</td> |
| <td>50</td> |
| </tr> |
| <tr> |
| <td>druid.query.scan.legacy</td> |
| <td>Whether legacy mode should be turned on for Scan queries</td> |
| <td>true or false</td> |
| <td>false</td> |
| </tr> |
| </tbody></table> |
| |
| <h2 id="query-context-properties">Query Context Properties</h2> |
| |
| <table><thead> |
| <tr> |
| <th>property</th> |
| <th>description</th> |
| <th>values</th> |
| <th>default</th> |
| </tr> |
| </thead><tbody> |
| <tr> |
| <td>maxRowsQueuedForOrdering</td> |
| <td>The maximum number of rows returned when time ordering is used. Overrides the identically named config.</td> |
| <td>An integer in [1, 2147483647]</td> |
| <td><code>druid.query.scan.maxRowsQueuedForOrdering</code></td> |
| </tr> |
| <tr> |
| <td>maxSegmentPartitionsOrderedInMemory</td> |
| <td>The maximum number of segments scanned per historical when time ordering is used. Overrides the identically named config.</td> |
| <td>An integer in [1, 2147483647]</td> |
| <td><code>druid.query.scan.maxSegmentPartitionsOrderedInMemory</code></td> |
| </tr> |
| </tbody></table> |
| |
| <p>Sample query context JSON object:</p> |
| <div class="highlight"><pre><code class="language-json" data-lang="json"><span></span><span class="p">{</span> |
| <span class="nt">"maxRowsQueuedForOrdering"</span><span class="p">:</span> <span class="mi">100001</span><span class="p">,</span> |
| <span class="nt">"maxSegmentPartitionsOrderedInMemory"</span><span class="p">:</span> <span class="mi">100</span> |
| <span class="p">}</span> |
| </code></pre></div> |
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