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| <div class="subTitle">org.apache.drill.exec.physical.impl.scan.project</div> |
| <h2 title="Class SchemaSmoother" class="title">Class SchemaSmoother</h2> |
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| <li>org.apache.drill.exec.physical.impl.scan.project.SchemaSmoother</li> |
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| <pre>public class <span class="typeNameLabel">SchemaSmoother</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">Implements a "schema smoothing" algorithm. |
| Schema persistence for the wildcard selection (i.e. SELECT *) |
| <p> |
| Constraints: |
| <ul> |
| <li>Adding columns causes a hard schema change.</li> |
| <li>Removing columns is allowed, uses type from previous |
| schema, as long as previous mode was nullable or repeated.</li> |
| <li>Changing type or mode causes a hard schema change.</li> |
| <li>Changing column order is fine; use order from previous |
| schema.</li> |
| </ul> |
| This can all be boiled down to a simpler rule: |
| <ul> |
| <li>Schema persistence is possible if the output schema |
| from a prior schema can be reused for the current schema.</li> |
| <li>Else, a hard schema change occurs and a new output |
| schema is derived from the new table schema.</li> |
| </ul> |
| The core idea here is to "unresolve" a fully-resolved table schema |
| to produce a new projection list that is the equivalent of using that |
| prior projection list in the SELECT. Then, keep that projection list only |
| if it is compatible with the next table schema, else throw it away and |
| start over from the actual scan projection list. |
| <p> |
| Algorithm: |
| <ul> |
| <li>If partitions are included in the wildcard, and the new |
| file needs more than the current one, create a new schema.</li> |
| <li>Else, treat partitions as select, fill in missing with |
| nulls.</li> |
| <li>From an output schema, construct a new select list |
| specification as though the columns in the current schema were |
| explicitly specified in the SELECT clause.</li> |
| <li>For each new schema column, verify that the column exists |
| in the generated SELECT clause and is of the same type. |
| If not, create a new schema.</li> |
| <li>Use the generated schema to plan a new projection from |
| the new schema to the prior schema.</li> |
| </ul></div> |
| </li> |
| </ul> |
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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/project/SchemaSmoother.IncompatibleSchemaException.html" title="class in org.apache.drill.exec.physical.impl.scan.project">SchemaSmoother.IncompatibleSchemaException</a></span></code> |
| <div class="block">Exception thrown if the prior schema is not compatible with the |
| new table schema.</div> |
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| <td class="colOne"><code><span class="memberNameLink"><a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/SchemaSmoother.html#SchemaSmoother-org.apache.drill.exec.physical.impl.scan.project.ScanLevelProjection-java.util.List-">SchemaSmoother</a></span>(<a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/ScanLevelProjection.html" title="class in org.apache.drill.exec.physical.impl.scan.project">ScanLevelProjection</a> scanProj, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/util/List.html?is-external=true" title="class or interface in java.util">List</a><<a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/ReaderLevelProjection.ReaderProjectionResolver.html" title="interface in org.apache.drill.exec.physical.impl.scan.project">ReaderLevelProjection.ReaderProjectionResolver</a>> resolvers)</code> </td> |
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| <td class="colFirst"><code><a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/ReaderLevelProjection.html" title="class in org.apache.drill.exec.physical.impl.scan.project">ReaderLevelProjection</a></code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/SchemaSmoother.html#resolve-org.apache.drill.exec.record.metadata.TupleMetadata-org.apache.drill.exec.physical.impl.scan.project.ResolvedTuple-">resolve</a></span>(<a href="../../../../../../../../org/apache/drill/exec/record/metadata/TupleMetadata.html" title="interface in org.apache.drill.exec.record.metadata">TupleMetadata</a> tableSchema, |
| <a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/ResolvedTuple.html" title="class in org.apache.drill.exec.physical.impl.scan.project">ResolvedTuple</a> outputTuple)</code> </td> |
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| <tr id="i1" class="rowColor"> |
| <td class="colFirst"><code>int</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/SchemaSmoother.html#schemaVersion--">schemaVersion</a></span>()</code> </td> |
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| <a name="SchemaSmoother-org.apache.drill.exec.physical.impl.scan.project.ScanLevelProjection-java.util.List-"> |
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| <h4>SchemaSmoother</h4> |
| <pre>public SchemaSmoother(<a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/ScanLevelProjection.html" title="class in org.apache.drill.exec.physical.impl.scan.project">ScanLevelProjection</a> scanProj, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/util/List.html?is-external=true" title="class or interface in java.util">List</a><<a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/ReaderLevelProjection.ReaderProjectionResolver.html" title="interface in org.apache.drill.exec.physical.impl.scan.project">ReaderLevelProjection.ReaderProjectionResolver</a>> resolvers)</pre> |
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| <h4>resolve</h4> |
| <pre>public <a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/ReaderLevelProjection.html" title="class in org.apache.drill.exec.physical.impl.scan.project">ReaderLevelProjection</a> resolve(<a href="../../../../../../../../org/apache/drill/exec/record/metadata/TupleMetadata.html" title="interface in org.apache.drill.exec.record.metadata">TupleMetadata</a> tableSchema, |
| <a href="../../../../../../../../org/apache/drill/exec/physical/impl/scan/project/ResolvedTuple.html" title="class in org.apache.drill.exec.physical.impl.scan.project">ResolvedTuple</a> outputTuple)</pre> |
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| <h4>schemaVersion</h4> |
| <pre>public int schemaVersion()</pre> |
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