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/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/**
* This package provides a "dummy" set of writers. The dummy writers provide
* the same API as the "real" writers, but the dummy writers simply discard
* their data. The dummy writers are used when implementing projection:
* non-projected columns may still have to be processed (as in a CSV file,
* say), but their values are not needed. One way to do this is to do an
* if-statement for each value:<pre><code>
* if (column-a-is-projected) {
* aWriter.setSomething(value);
* }</code></pre>
* The dummy writers convert the if-statement into a virtual function call,
* same as is done to handle the type-specific nature of vectors:
* <pre><code>
* aWriter.setSomething(value);
* </code></pre>
* <p>
* The theory is that the virtual function dispatch is simpler, and faster,
* than doing continual if-checks everywhere in the code.
* <p>
* The dummy writers reside in this package so that the various factory
* methods can automatically build the dummy versions when given a null
* value vector (which we then interpret to mean that there is no physical
* backing to the column.)
* <p>
* At present, most methods that return a value simply return zero or
* null.
* Experience will show whether it is worthwhile implementing some
* basics, such as a value type or index. For now, these return null,
* assuming that the caller won't do anything with the column other
* than set a value.
* <p>
* Some simpler dummy writers appear as nested classes inside the
* "real" writers.
*/
package org.apache.drill.exec.vector.accessor.writer.dummy;
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