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| <div class="subTitle">org.apache.drill.exec.physical.impl.protocol</div> |
| <h2 title="Class SchemaTracker" class="title">Class SchemaTracker</h2> |
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| <ul class="inheritance"> |
| <li><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">java.lang.Object</a></li> |
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| <pre>public class <span class="typeNameLabel">SchemaTracker</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></pre> |
| <div class="block">Tracks changes to schemas via "snapshots" over time. That is, given |
| a schema, tracks if a new schema is the same as the current one. For |
| example, each batch output from a series of readers might be compared, |
| as they are returned, to detect schema changes from one batch to the |
| next. This class does not track vector-by-vector changes as a schema |
| is built, but rather periodic "snapshots" at times determined by the |
| operator. |
| <p> |
| If an operator is guaranteed to emit a consistent schema, then no |
| checks need be done, and this tracker will report no schema change. |
| On the other hand, a scanner might check schema more often. At least |
| once per reader, and more often if a reader is "late-schema": if the |
| reader can change schema batch-by-batch. |
| <p> |
| Drill defines "schema change" in a very specific way. Not only must |
| the set of columns be the same, and have the same types, it must also |
| be the case that the <b>vectors</b> that hold the columns be identical. |
| Generated code contains references to specific vector objects; passing |
| along different vectors requires new code to be generated and is treated |
| as a schema change. |
| <p> |
| Drill has no concept of "same schema, different vectors." A change in |
| vector is just as serious as a change in schema. Hence, operators |
| try to use the same vectors for their entire lives. That is the change |
| tracked here. |
| <p> |
| Schema versions start at 1. A schema version of 0 means that no |
| output batch was ever presented.</div> |
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| <td class="colOne"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/SchemaTracker.html#SchemaTracker--">SchemaTracker</a></span>()</code> </td> |
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| <td class="colFirst"><code><a href="../../../../../../../org/apache/drill/exec/record/BatchSchema.html" title="class in org.apache.drill.exec.record">BatchSchema</a></code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/SchemaTracker.html#schema--">schema</a></span>()</code> </td> |
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| <td class="colFirst"><code>int</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/SchemaTracker.html#schemaVersion--">schemaVersion</a></span>()</code> </td> |
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| <tr id="i2" class="altColor"> |
| <td class="colFirst"><code>void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/drill/exec/physical/impl/protocol/SchemaTracker.html#trackSchema-org.apache.drill.exec.record.VectorContainer-">trackSchema</a></span>(<a href="../../../../../../../org/apache/drill/exec/record/VectorContainer.html" title="class in org.apache.drill.exec.record">VectorContainer</a> newBatch)</code> </td> |
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| <h4>SchemaTracker</h4> |
| <pre>public SchemaTracker()</pre> |
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| <h4>trackSchema</h4> |
| <pre>public void trackSchema(<a href="../../../../../../../org/apache/drill/exec/record/VectorContainer.html" title="class in org.apache.drill.exec.record">VectorContainer</a> newBatch)</pre> |
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| <h4>schemaVersion</h4> |
| <pre>public int schemaVersion()</pre> |
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| <h4>schema</h4> |
| <pre>public <a href="../../../../../../../org/apache/drill/exec/record/BatchSchema.html" title="class in org.apache.drill.exec.record">BatchSchema</a> schema()</pre> |
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