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https://issues.apache.org/jira/browse/DRILL-6791?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16686534#comment-16686534
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ASF GitHub Bot commented on DRILL-6791:
---------------------------------------
arina-ielchiieva commented on a change in pull request #1501: DRILL-6791: Scan
projection framework
URL: https://github.com/apache/drill/pull/1501#discussion_r233450250
##########
File path:
exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/scan/project/MetadataManager.java
##########
@@ -0,0 +1,73 @@
+/*
+ * 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.impl.scan.project;
+
+import
org.apache.drill.exec.physical.impl.scan.project.ScanLevelProjection.ScanProjectionParser;
+import
org.apache.drill.exec.physical.impl.scan.project.SchemaLevelProjection.SchemaProjectionResolver;
+import org.apache.drill.exec.physical.rowSet.ResultVectorCache;
+
+/**
+ * Queries can contain a wildcard (*), table columns, or special
+ * system-defined columns (the file metadata columns AKA implicit
+ * columns, the `columns` column of CSV, etc.).
+ * <p>
+ * This class provides a generalized way of handling such extended
+ * columns. That is, this handles metadata for columns defined by
+ * the scan or file; columns defined by the table (the actual
+ * data metadata) is handled elsewhere.
+ * <p>
+ * Objects of this interface are driven by the projection processing
+ * framework which provides a vector cache from which to obtain
+ * materialized columns. The implementation must provide a projection
+ * parser to pick out the columns which this object handles.
+ * <p>
+ * A better name might be ImplicitMetadataManager to signify that
Review comment:
Agree, let's rename to avoid the confusion in future.
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> Merge scan projection framework into master
> -------------------------------------------
>
> Key: DRILL-6791
> URL: https://issues.apache.org/jira/browse/DRILL-6791
> Project: Apache Drill
> Issue Type: Improvement
> Affects Versions: 1.15.0
> Reporter: Paul Rogers
> Assignee: Paul Rogers
> Priority: Major
> Fix For: 1.15.0
>
>
> Merge the next set of "result set loader" code into master via a PR. This one
> covers the "schema projection" mechanism which:
> * Handles none (SELECT COUNT\(*)), some (SELECT a, b, x) and all (SELECT *)
> projection.
> * Handles null columns (for projection a column "x" that does not exist in
> the base table.)
> * Handles constant columns as used for file metadata (AKA "implicit" columns).
> * Handle schema persistence: the need to reuse the same vectors across
> different scanners
> * Provides a framework for consuming externally-supplied metadata
> * Since we don't yet have a way to provide "real" metadata, obtains metadata
> hints from previous batches and from the projection list (a.b implies that
> "a" is a map, c[0] implies that "c" is an array, etc.)
> * Handles merging the set of data source columns and null columns to create
> the final output batch.
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