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| <div class="subTitle">org.apache.drill.exec.physical.impl.scan</div> |
| <h2 title="Class ScanOperatorExec" class="title">Class ScanOperatorExec</h2> |
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| <li>org.apache.drill.exec.physical.impl.scan.ScanOperatorExec</li> |
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| <dl> |
| <dt>All Implemented Interfaces:</dt> |
| <dd><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html" title="interface in org.apache.drill.exec.physical.impl.protocol">OperatorExec</a></dd> |
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| <pre>public class <span class="typeNameLabel">ScanOperatorExec</span> |
| extends <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a> |
| implements <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html" title="interface in org.apache.drill.exec.physical.impl.protocol">OperatorExec</a></pre> |
| <div class="block">Implementation of the revised scan operator that uses a mutator aware of |
| batch sizes. This is the successor to <a href="../../../../../../../org/apache/drill/exec/physical/impl/ScanBatch.html" title="class in org.apache.drill.exec.physical.impl"><code>ScanBatch</code></a> and should be used |
| by all new scan implementations. |
| <p> |
| The basic concept is to split the scan operator into layers: |
| <ul> |
| <li>The <code>OperatorRecordBatch</code> which implements Drill's Volcano-like |
| protocol.</li> |
| <li>The scan operator "wrapper" (this class) which implements actions for the |
| operator record batch specifically for scan. It iterates over readers, |
| delegating semantic work to other classes.</li> |
| <li>The implementation of per-reader semantics in the two EVF versions and |
| other ad-hoc implementations.</li> |
| <li>The result set loader and related classes which pack values into |
| value vectors.</li> |
| <li>Value vectors, which store the data.</li> |
| </ul> |
| <p> |
| The layered format can be confusing. However, each layer is somewhat |
| complex, so dividing the work into layers keeps the overall complexity |
| somewhat under control. |
| |
| <h4>Scanner Framework</h4> |
| |
| Acts as an adapter between the operator protocol and the row reader |
| protocol. |
| <p> |
| The scan operator itself is simply a framework for handling a set of readers; |
| it knows nothing other than the interfaces of the components it works with; |
| delegating all knowledge of schemas, projection, reading and the like to |
| implementations of those interfaces. Because that work is complex, a set |
| of frameworks exist to handle most common use cases, but a specialized reader |
| can create a framework or reader from scratch. |
| <p> |
| Error handling in this class is minimal: the enclosing record batch iterator |
| is responsible for handling exceptions. Error handling relies on the fact |
| that the iterator will call <tt>close()</tt> regardless of which exceptions |
| are thrown. |
| |
| <h4>Protocol</h4> |
| |
| The scanner works directly with two other interfaces |
| <p> |
| The <a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorEvents.html" title="interface in org.apache.drill.exec.physical.impl.scan"><code>ScanOperatorEvents</code></a> implementation provides the set of readers to |
| use. This class can simply maintain a list, or can create the reader on |
| demand. |
| <p> |
| More subtly, the factory also handles projection issues and manages vectors |
| across subsequent readers. A number of factories are available for the most |
| common cases. Extend these to implement a version specific to a data source. |
| <p> |
| The <a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/RowBatchReader.html" title="interface in org.apache.drill.exec.physical.impl.scan"><code>RowBatchReader</code></a> is a surprisingly minimal interface that |
| nonetheless captures the essence of reading a result set as a set of batches. |
| The factory implementations mentioned above implement this interface to provide |
| commonly-used services, the most important of which is access to a |
| {#link ResultSetLoader} to write values into value vectors. |
| |
| <h4>Schema Versions</h4> |
| |
| Readers may change schemas from time to time. To track such changes, |
| this implementation tracks a batch schema version, maintained by comparing |
| one schema with the next. |
| <p> |
| Readers can discover columns as they read data, such as with any JSON-based |
| format. In this case, the row set mutator also provides a schema version, |
| but a fine-grained one that changes each time a column is added. |
| <p> |
| The two schema versions serve different purposes and are not interchangeable. |
| For example, if a scan reads two files, both will build up their own schemas, |
| each increasing its internal version number as work proceeds. But, at the |
| end of each batch, the schemas may (and, in fact, should) be identical, |
| which is the schema version downstream operators care about. |
| |
| <h4>Empty Files and/or Empty Schemas</h4> |
| |
| A corner case occurs if the input is empty, such as a CSV file |
| that contains no data. The general rule is the following: |
| |
| <ul> |
| <li>If the reader is "early schema" (the schema is defined at |
| open time), then the result will be a single empty batch with |
| the schema defined. Example: a CSV file without headers; in this case, |
| we know the schema is always the single `columns` array.</li> |
| <li>If the reader is "late schema" (the schema is defined while the |
| data is read), then no batch is returned because there is no schema. |
| Example: a JSON file. It is not helpful to return a single batch |
| with no columns; such a batch will simply conflict with some other |
| non-empty-schema batch. It turns out that other DBs handle this |
| case gracefully: a query of the form<br><pre><tt> |
| SELECT * FROM VALUES()</tt></pre><br> |
| Will produce an empty result: no schema, no data.</li> |
| <li>The hybrid case: the reader could provide an early schema, |
| but cannot do so. That is, the early schema contains no columns. |
| We treat this case identically to the late schema case. Example: a |
| CSV file with headers in which the header line is empty.</li> |
| </ul></div> |
| </li> |
| </ul> |
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| <td class="colFirst"><code>protected <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/VectorContainerAccessor.html" title="class in org.apache.drill.exec.physical.impl.protocol">VectorContainerAccessor</a></code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorExec.html#containerAccessor">containerAccessor</a></span></code> </td> |
| </tr> |
| <tr class="rowColor"> |
| <td class="colFirst"><code>protected <a href="../../../../../../../org/apache/drill/exec/ops/OperatorContext.html" title="interface in org.apache.drill.exec.ops">OperatorContext</a></code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorExec.html#context">context</a></span></code> </td> |
| </tr> |
| </table> |
| </li> |
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| <th class="colOne" scope="col">Constructor and Description</th> |
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| <tr class="altColor"> |
| <td class="colOne"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorExec.html#ScanOperatorExec-org.apache.drill.exec.physical.impl.scan.ScanOperatorEvents-boolean-">ScanOperatorExec</a></span>(<a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorEvents.html" title="interface in org.apache.drill.exec.physical.impl.scan">ScanOperatorEvents</a> factory, |
| boolean allowEmptyResult)</code> </td> |
| </tr> |
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| <tr> |
| <th class="colFirst" scope="col">Modifier and Type</th> |
| <th class="colLast" scope="col">Method and Description</th> |
| </tr> |
| <tr id="i0" class="altColor"> |
| <td class="colFirst"><code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/BatchAccessor.html" title="interface in org.apache.drill.exec.physical.impl.protocol">BatchAccessor</a></code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorExec.html#batchAccessor--">batchAccessor</a></span>()</code> |
| <div class="block">Provides a generic access mechanism to the batch's output data.</div> |
| </td> |
| </tr> |
| <tr id="i1" class="rowColor"> |
| <td class="colFirst"><code>void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorExec.html#bind-org.apache.drill.exec.ops.OperatorContext-">bind</a></span>(<a href="../../../../../../../org/apache/drill/exec/ops/OperatorContext.html" title="interface in org.apache.drill.exec.ops">OperatorContext</a> context)</code> |
| <div class="block">Bind this operator to the context.</div> |
| </td> |
| </tr> |
| <tr id="i2" class="altColor"> |
| <td class="colFirst"><code>boolean</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorExec.html#buildSchema--">buildSchema</a></span>()</code> |
| <div class="block">Retrieves the schema of the batch before the first actual batch |
| of data.</div> |
| </td> |
| </tr> |
| <tr id="i3" class="rowColor"> |
| <td class="colFirst"><code>void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorExec.html#cancel--">cancel</a></span>()</code> |
| <div class="block">Alerts the operator that the query was cancelled.</div> |
| </td> |
| </tr> |
| <tr id="i4" class="altColor"> |
| <td class="colFirst"><code>void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorExec.html#close--">close</a></span>()</code> |
| <div class="block">Close the operator by releasing all resources that the operator |
| held.</div> |
| </td> |
| </tr> |
| <tr id="i5" class="rowColor"> |
| <td class="colFirst"><code><a href="../../../../../../../org/apache/drill/exec/ops/OperatorContext.html" title="interface in org.apache.drill.exec.ops">OperatorContext</a></code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorExec.html#context--">context</a></span>()</code> </td> |
| </tr> |
| <tr id="i6" class="altColor"> |
| <td class="colFirst"><code>boolean</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorExec.html#next--">next</a></span>()</code> |
| <div class="block">Retrieves the next batch of data.</div> |
| </td> |
| </tr> |
| </table> |
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| </ul> |
| </li> |
| </ul> |
| </li> |
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| <h3>Field Detail</h3> |
| <a name="containerAccessor"> |
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| <h4>containerAccessor</h4> |
| <pre>protected final <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/VectorContainerAccessor.html" title="class in org.apache.drill.exec.physical.impl.protocol">VectorContainerAccessor</a> containerAccessor</pre> |
| </li> |
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| <a name="context"> |
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| <li class="blockList"> |
| <h4>context</h4> |
| <pre>protected <a href="../../../../../../../org/apache/drill/exec/ops/OperatorContext.html" title="interface in org.apache.drill.exec.ops">OperatorContext</a> context</pre> |
| </li> |
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| <h3>Constructor Detail</h3> |
| <a name="ScanOperatorExec-org.apache.drill.exec.physical.impl.scan.ScanOperatorEvents-boolean-"> |
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| </a> |
| <ul class="blockListLast"> |
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| <h4>ScanOperatorExec</h4> |
| <pre>public ScanOperatorExec(<a href="../../../../../../../org/apache/drill/exec/physical/impl/scan/ScanOperatorEvents.html" title="interface in org.apache.drill.exec.physical.impl.scan">ScanOperatorEvents</a> factory, |
| boolean allowEmptyResult)</pre> |
| </li> |
| </ul> |
| </li> |
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| <a name="bind-org.apache.drill.exec.ops.OperatorContext-"> |
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| <h4>bind</h4> |
| <pre>public void bind(<a href="../../../../../../../org/apache/drill/exec/ops/OperatorContext.html" title="interface in org.apache.drill.exec.ops">OperatorContext</a> context)</pre> |
| <div class="block"><span class="descfrmTypeLabel">Description copied from interface: <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#bind-org.apache.drill.exec.ops.OperatorContext-">OperatorExec</a></code></span></div> |
| <div class="block">Bind this operator to the context. The context provides access |
| to per-operator, per-fragment and per-Drillbit services. |
| Also provides access to the operator definition (AKA "pop |
| config") for this operator.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Specified by:</span></dt> |
| <dd><code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#bind-org.apache.drill.exec.ops.OperatorContext-">bind</a></code> in interface <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html" title="interface in org.apache.drill.exec.physical.impl.protocol">OperatorExec</a></code></dd> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>context</code> - operator context</dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="batchAccessor--"> |
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| </a> |
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| <h4>batchAccessor</h4> |
| <pre>public <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/BatchAccessor.html" title="interface in org.apache.drill.exec.physical.impl.protocol">BatchAccessor</a> batchAccessor()</pre> |
| <div class="block"><span class="descfrmTypeLabel">Description copied from interface: <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#batchAccessor--">OperatorExec</a></code></span></div> |
| <div class="block">Provides a generic access mechanism to the batch's output data. |
| This method is called after a successful return from |
| <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#buildSchema--"><code>OperatorExec.buildSchema()</code></a> and <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#next--"><code>OperatorExec.next()</code></a>. The batch itself |
| can be held in a standard <code>VectorContainer</code>, or in some |
| other structure more convenient for this operator.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Specified by:</span></dt> |
| <dd><code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#batchAccessor--">batchAccessor</a></code> in interface <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html" title="interface in org.apache.drill.exec.physical.impl.protocol">OperatorExec</a></code></dd> |
| <dt><span class="returnLabel">Returns:</span></dt> |
| <dd>the access for the batch's output container</dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="context--"> |
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| </a> |
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| <li class="blockList"> |
| <h4>context</h4> |
| <pre>public <a href="../../../../../../../org/apache/drill/exec/ops/OperatorContext.html" title="interface in org.apache.drill.exec.ops">OperatorContext</a> context()</pre> |
| </li> |
| </ul> |
| <a name="buildSchema--"> |
| <!-- --> |
| </a> |
| <ul class="blockList"> |
| <li class="blockList"> |
| <h4>buildSchema</h4> |
| <pre>public boolean buildSchema()</pre> |
| <div class="block"><span class="descfrmTypeLabel">Description copied from interface: <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#buildSchema--">OperatorExec</a></code></span></div> |
| <div class="block">Retrieves the schema of the batch before the first actual batch |
| of data. The schema is returned via an empty batch (no rows, |
| only schema) from <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#batchAccessor--"><code>OperatorExec.batchAccessor()</code></a>.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Specified by:</span></dt> |
| <dd><code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#buildSchema--">buildSchema</a></code> in interface <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html" title="interface in org.apache.drill.exec.physical.impl.protocol">OperatorExec</a></code></dd> |
| <dt><span class="returnLabel">Returns:</span></dt> |
| <dd>true if a schema is available, false if the operator |
| reached EOF before a schema was found</dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="next--"> |
| <!-- --> |
| </a> |
| <ul class="blockList"> |
| <li class="blockList"> |
| <h4>next</h4> |
| <pre>public boolean next()</pre> |
| <div class="block"><span class="descfrmTypeLabel">Description copied from interface: <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#next--">OperatorExec</a></code></span></div> |
| <div class="block">Retrieves the next batch of data. The data is returned via |
| the <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#batchAccessor--"><code>OperatorExec.batchAccessor()</code></a> method.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Specified by:</span></dt> |
| <dd><code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#next--">next</a></code> in interface <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html" title="interface in org.apache.drill.exec.physical.impl.protocol">OperatorExec</a></code></dd> |
| <dt><span class="returnLabel">Returns:</span></dt> |
| <dd>true if another batch of data is available, false if |
| EOF was reached and no more data is available</dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="cancel--"> |
| <!-- --> |
| </a> |
| <ul class="blockList"> |
| <li class="blockList"> |
| <h4>cancel</h4> |
| <pre>public void cancel()</pre> |
| <div class="block"><span class="descfrmTypeLabel">Description copied from interface: <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#cancel--">OperatorExec</a></code></span></div> |
| <div class="block">Alerts the operator that the query was cancelled. Generally |
| optional, but allows the operator to realize that a cancellation |
| was requested.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Specified by:</span></dt> |
| <dd><code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#cancel--">cancel</a></code> in interface <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html" title="interface in org.apache.drill.exec.physical.impl.protocol">OperatorExec</a></code></dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="close--"> |
| <!-- --> |
| </a> |
| <ul class="blockListLast"> |
| <li class="blockList"> |
| <h4>close</h4> |
| <pre>public void close()</pre> |
| <div class="block"><span class="descfrmTypeLabel">Description copied from interface: <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#close--">OperatorExec</a></code></span></div> |
| <div class="block">Close the operator by releasing all resources that the operator |
| held. Called after <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#cancel--"><code>OperatorExec.cancel()</code></a> and after <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#batchAccessor--"><code>OperatorExec.batchAccessor()</code></a> |
| or <a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#next--"><code>OperatorExec.next()</code></a> returns false. |
| <p> |
| Note that there may be a significant delay between the last call to |
| <tt>next()</tt> and the call to <tt>close()</tt> during which |
| downstream operators do their work. A tidy operator will release |
| resources immediately after EOF to avoid holding onto memory or other |
| resources that could be used by downstream operators.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Specified by:</span></dt> |
| <dd><code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html#close--">close</a></code> in interface <code><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/OperatorExec.html" title="interface in org.apache.drill.exec.physical.impl.protocol">OperatorExec</a></code></dd> |
| </dl> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
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