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| <h2 title="Interface ManagedReader" class="title">Interface ManagedReader</h2> |
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| <pre>public interface <span class="typeNameLabel">ManagedReader</span></pre> |
| <div class="block">Extended version of a record reader which uses a size-aware batch mutator. |
| Use this for all new readers. Replaces the original |
| <a href="../../../../../../../../org/apache/drill/exec/store/RecordReader.html" title="interface in org.apache.drill.exec.store"><code>RecordReader</code></a> interface. |
| <p> |
| This interface is used to create readers that work with the projection |
| mechanism to provide services for handling projection, setting up the result |
| set loader, handling schema smoothing, sharing vectors across batches, etc. |
| <p> |
| Note that this interface reads a <b>batch</b> of rows, not a single row. (The |
| original <code>RecordReader</code> could be confusing in this aspect.) |
| <p> |
| The expected lifecycle is: |
| <ul> |
| <li>The reader factory creates the reader just before using it. (Unlike |
| the old <code>ScanBatch</code> which created all readers at the start of the |
| scan.)</li> |
| <li>Constructor: open the reader using the |
| <a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/v3/SchemaNegotiator.html" title="interface in org.apache.drill.exec.physical.impl.scan.v3"><code>SchemaNegotiator</code></a> to configure the |
| scanner framework for this reader by specifying a schema (if known), desired |
| row counts and other configuration options. Call <a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/v3/SchemaNegotiator.html#build--"><code>SchemaNegotiator.build()</code></a> |
| to obtain a <a href="../../../../../../../../org/apache/drill/exec/physical/resultSet/RowSetLoader.html" title="interface in org.apache.drill.exec.physical.resultSet"><code>RowSetLoader</code></a> |
| to use to capture the rows that the reader reads.</li> |
| <li><a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/v3/ManagedReader.html#next--"><code>next()</code></a>: called for each batch. The batch is written using the |
| result set loader obtained above. The scanner framework handles details of |
| tracking version changes, handling overflow, limiting record counts, and |
| so on. Return <tt>true</tt> to indicate a batch is available, <tt>false</tt> |
| to indicate EOF. The first call to <tt>next()</tt> can return <tt>false</tt> |
| if the data source has no rows.</li> |
| <li><a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/v3/ManagedReader.html#close--"><code>close()</code></a>: called to release resources. May be called before |
| <tt>next()</tt> returns </tt>false</tt>.</li> |
| <p> |
| If an error occurs, the reader can throw a <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/RuntimeException.html?is-external=true" title="class or interface in java.lang"><code>RuntimeException</code></a> |
| from any method. A <tt>UserException</tt> is preferred to provide |
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| <td class="colFirst"><code>static class </code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/v3/ManagedReader.EarlyEofException.html" title="class in org.apache.drill.exec.physical.impl.scan.v3">ManagedReader.EarlyEofException</a></span></code> |
| <div class="block">Exception thrown from the constructor if the data source is empty and |
| can produce no data or schema.</div> |
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| <td class="colFirst"><code>void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/v3/ManagedReader.html#close--">close</a></span>()</code> |
| <div class="block">Release resources.</div> |
| </td> |
| </tr> |
| <tr id="i1" class="rowColor"> |
| <td class="colFirst"><code>boolean</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/v3/ManagedReader.html#next--">next</a></span>()</code> |
| <div class="block">Read the next batch.</div> |
| </td> |
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| <h4>next</h4> |
| <pre>boolean next()</pre> |
| <div class="block">Read the next batch. Reading continues until either EOF, |
| or until the mutator indicates that the batch is full. |
| The batch is considered valid if it is non-empty. Returning |
| <tt>true</tt> with an empty batch is valid, and is helpful on |
| the very first batch (returning schema only.) An empty batch |
| with a <tt>false</tt> return code indicates EOF and the batch |
| will be discarded. A non-empty batch along with a <tt>false</tt> |
| return result indicates a final, valid batch, but that EOF was |
| reached and no more data is available. |
| <p> |
| This somewhat complex protocol avoids the need to allocate a |
| final batch just to find out that no more data is available; |
| it allows EOF to be returned along with the final batch.</div> |
| <dl> |
| <dt><span class="returnLabel">Returns:</span></dt> |
| <dd><tt>true</tt> if more data may be available (and so |
| <tt>next()</tt> should be called again, <tt>false</tt> to indicate |
| that EOF was reached</dd> |
| <dt><span class="throwsLabel">Throws:</span></dt> |
| <dd><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/RuntimeException.html?is-external=true" title="class or interface in java.lang">RuntimeException</a></code> - (<tt>UserException</tt> preferred) if an |
| error occurs that should fail the query.</dd> |
| </dl> |
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| <h4>close</h4> |
| <pre>void close()</pre> |
| <div class="block">Release resources. Called just after a failure, when the scanner |
| is cancelled, or after <tt>next()</tt> returns EOF. Release |
| all resources and close files. Guaranteed to be called if |
| <tt>open()</tt> returns normally; will not be called if <tt>open()</tt> |
| throws an exception.</div> |
| <dl> |
| <dt><span class="throwsLabel">Throws:</span></dt> |
| <dd><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/RuntimeException.html?is-external=true" title="class or interface in java.lang">RuntimeException</a></code> - (<tt>UserException</tt> preferred) if an |
| error occurs that should fail the query.</dd> |
| </dl> |
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