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https://issues.apache.org/jira/browse/DRILL-7278?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16854857#comment-16854857
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ASF GitHub Bot commented on DRILL-7278:
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arina-ielchiieva commented on pull request #1797: DRILL-7278: Refactor result
set loader projection mechanism
URL: https://github.com/apache/drill/pull/1797#discussion_r289942694
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File path:
exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/scan/project/projSet/package-info.java
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@@ -0,0 +1,98 @@
+/*
+ * 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.
+ */
+
+/**
+ * The dynamic projection in Drill is complex. With the advent of
+ * provided schema, we now have many ways to manage projection. The
+ * classes here implement these many policies. They are implemnted
+ * as distinct classes (rather than chains of if-statements) to
+ * make the classes easier to test and reason about.
+ * <p>
+ * Projection is a combination of three distinct policies:
+ * <ul>
+ * <li>Projection policy (all, none, explicit, etc.)</li>
+ * <li>Column policy (unprojected, explicit projection,
+ * projection with schema, etc.)</li>
+ * <li>Type conversion: none, based on a provided schema,
+ * custom.</li>
+ * </ul>
+ * Experience has shown that these must be separated: each is designed
+ * and tested separately to keep the problem tractable.
+ *
+ * <h4>Projection Set Cases</h4>
+ *
+ * The project cases and their classes:
+ * <p>
+ * <dl>
+ * <dt>{@link EmptyProjectionSet}</dt>
+ * <dd><tt>SELECT COUNT(*)</tt>: Project nothing. Only count records.</dd>
+ * <dl>
+ * <dt>{@link WildcardProjectionSet}</dt>
+ * <dd><tt>SELECT *</tt>: Project everything, with an optional provided
+ * schema. If a schema is provided, and is strict, then project only
+ * reader columns that appear in the provided schema.
+ * However, don't project columns which have been marked as
+ * special: {@link ColumnMetadata#EXCLUDE_FROM_WILDCARD}, whether marked
+ * in the reader or provided schemas.</dd>
+ * <dt>{@link ExplicitProjectionSet}</dt>
+ * <dd><tt>SELECT a, b[10], c.d</tt>: Explicit projection with or without
+ * a schema. Project only the selected columns. Verify that the reader
+ * provides column types/modes consistent with the implied form in the
+ * projection list. That is, in this example, `b` must be an array.</dd>
+ * </dl>
+ *
+ * <h4>Column Projection Cases</h4>
+ *
+ * Each projection set answers a query: "the reader wants to add such-and-so
+ * column: what should I do?" Since the reader is free to add any column,
+ * we don't cache the list of columns as is done with the parsed project
+ * list, or the output schema. Instead, we handle each column on a
+ * case-by-case basis; we create a {@link ColumnReadProjection} instance
+ * to answer the query. Instances of this class are meant to be transient:
+ * use them and discard them. We answer the query differently depending on
+ * many factors, including:
+ * <p>
+ * <dl>
+ * <dt>{@link UnprojectedReadColumn}</dt>
+ * <dd>Column is not projected. Nothing to convert, no type checks
+ * needed. The result set loader should create a dummy writer for this
+ * case.</dd>
+ * <dt>{@link ProjectedReadColumn}</dt>
+ * <dd>Column is projected. It may have an associated projection list
+ * item, an output schema, or a type conversion. All these variations
+ * should be transparent to the consumer.</dd>
+ * </dl>
+ *
+ * <h4>Type Conversion</h4>
+ *
+ * The {@link TypeConverter} class handles a provided schema, custom type
+ * conversion, and custom properties passed to the conversion shims. A null
+ * type converter passed to a projection set means no conversion is done.
Review comment:
Same here - should we use some dummy converter to avoid dealing with nulls?
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> Refactor result set loader projection mechanism
> -----------------------------------------------
>
> Key: DRILL-7278
> URL: https://issues.apache.org/jira/browse/DRILL-7278
> Project: Apache Drill
> Issue Type: Improvement
> Affects Versions: 1.16.0
> Reporter: Paul Rogers
> Assignee: Paul Rogers
> Priority: Major
> Fix For: 1.17.0
>
>
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