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https://issues.apache.org/jira/browse/DRILL-7278?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16854849#comment-16854849
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ASF GitHub Bot commented on DRILL-7278:
---------------------------------------
arina-ielchiieva commented on pull request #1797: DRILL-7278: Refactor result
set loader projection mechanism
URL: https://github.com/apache/drill/pull/1797#discussion_r289943217
##########
File path:
exec/java-exec/src/main/java/org/apache/drill/exec/physical/rowSet/ProjectionSet.java
##########
@@ -0,0 +1,105 @@
+/*
+ * 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.
+ */
+package org.apache.drill.exec.physical.rowSet;
+
+import org.apache.drill.common.exceptions.CustomErrorContext;
+import org.apache.drill.exec.physical.rowSet.project.ProjectionType;
+import org.apache.drill.exec.record.metadata.ColumnMetadata;
+import org.apache.drill.exec.vector.accessor.convert.ColumnConversionFactory;
+import
org.apache.drill.shaded.guava.com.google.common.annotations.VisibleForTesting;
+
+/**
+ * Provides a dynamic, run-time view of a projection set. Used by
+ * the result set loader to:
+ * <ul>
+ * <li>Determine if a column is projected according to some
+ * defined projection schema (see implementation for details.)</li>
+ * <li>Provide type conversions, either using built-in implicit
+ * conversions, or a custom conversion. Type conversions require
+ * the reader column and a "provided" column that gives the "to"
+ * type for the conversion. Without the "to" column, the reader
+ * column type is used as-is.</li>
+ * <li>Verify that the (possibly converted) type and mode are
+ * compatible with an explicit projection item. For example, if
+ * the query has `a.b`, but `a` is scalar, then there is an
+ * inconsistency.</li>
+ * </ul>
+ * <p>
+ * This interface filters columns added dynamically
+ * at scan time. The reader may offer a column (as to add a column
+ * writer for the column.) The projection mechanism says whether to
+ * materialize the column, or whether to ignore the column and
+ * return a dummy column writer.
+ * <p>
+ * The Project All must handle several additional nuances:
+ * <ul>
+ * <li>External schema: If an external schema is provided, then that
+ * schema may be "strict" which causes the wildcard to expand to the
+ * set of columns defined within the schema. When used with columns
+ * added dynamically, a column may be excluded from the projection
+ * set if it is not part of the defined external schema.</ul>
+ * <li>Metadata filtering: A reader may offer a special column which
+ * is available only in explicit projection, and behaves like Drill's
+ * implicit file columns. Such columns are not included in a "project
+ * all" projection.</li>
+ * <p>
+ * At present, only the top-level row supports these additional filtering
+ * options; they are not supported on mays (though could be with additional
Review comment:
```suggestion
* options; they are not supported on maps (though could be with additional
```
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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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