arina-ielchiieva commented on a change in 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 ``` ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: us...@infra.apache.org With regards, Apache Git Services
