| /* |
| * 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 java.util.ArrayList; |
| import java.util.List; |
| |
| import org.apache.drill.common.types.TypeProtos.DataMode; |
| import org.apache.drill.exec.physical.impl.scan.project.SchemaSmoother.IncompatibleSchemaException; |
| import org.apache.drill.exec.record.MaterializedField; |
| import org.apache.drill.exec.record.metadata.TupleMetadata; |
| |
| /** |
| * Resolve a table schema against the prior schema. This works only if the |
| * types match and if all columns in the table schema already appear in the |
| * prior schema. |
| * <p> |
| * Consider this an experimental mechanism. The hope was that, with clever |
| * techniques, we could "smooth over" some of the issues that cause schema |
| * change events in Drill. As it turned out, however, creating this mechanism |
| * revealed that it is not possible, even in theory, to handle most schema |
| * changes because of the time dimension: |
| * <ul> |
| * <li>An even in a later batch may provide information that would have |
| * caused us to make a different decision in an earlier batch. For example, |
| * we are asked for column `foo`, did not see such a column in the first |
| * batch, block or file, guessed some type, and later saw that the column |
| * was of a different type. We can't "time travel" to tell our earlier |
| * selves, nor, when we make the initial type decision, can we jump to |
| * the future to see what type we'll discover.</li> |
| * <li>Readers in this fragment may see column `foo` but readers in |
| * another fragment read files/blocks that don't have that column. The |
| * two readers cannot communicate to agree on a type.</li> |
| * </ul> |
| * <p> |
| * What this mechanism can do is make decisions based on history: when a |
| * column appears, we can adjust its type a bit to try to avoid an |
| * unnecessary change. For example, if a prior file in this scan saw |
| * `foo` as nullable Varchar, but the present file has the column as |
| * requied Varchar, we can use the more general nullable form. But, |
| * again, the "can't predict the future" bites us: we can handle a |
| * nullable-to-required column change, but not visa-versa. |
| * <p> |
| * What this mechanism will tell the careful reader is that the only |
| * general solution to the schema-change problem is to now the full |
| * schema up front: for the planner to be told the schema and to |
| * communicate that schema to all readers so that all readers agree |
| * on the final schema. |
| * <p> |
| * When that is done, the techniques shown here can be used to adjust |
| * any per-file variation of schema to match the up-front schema. |
| */ |
| |
| public class SmoothingProjection extends ReaderLevelProjection { |
| |
| protected final List<MaterializedField> rewrittenFields = new ArrayList<>(); |
| |
| public SmoothingProjection(ScanLevelProjection scanProj, |
| TupleMetadata tableSchema, |
| ResolvedTuple priorSchema, |
| ResolvedTuple outputTuple, |
| List<ReaderProjectionResolver> resolvers) throws IncompatibleSchemaException { |
| |
| super(resolvers); |
| |
| for (ResolvedColumn priorCol : priorSchema.columns()) { |
| if (priorCol instanceof ResolvedTableColumn || |
| priorCol instanceof ResolvedNullColumn) { |
| // This is a regular column known to this base framework. |
| resolveColumn(outputTuple, priorCol, tableSchema); |
| } else { |
| // The column is one known to an add-on mechanism. |
| resolveSpecial(outputTuple, priorCol, tableSchema); |
| } |
| } |
| |
| // Check if all table columns were matched. Since names are unique, |
| // each column can be matched at most once. If the number of matches is |
| // less than the total number of columns, then some columns were not |
| // matched and we must start over. |
| |
| if (rewrittenFields.size() < tableSchema.size()) { |
| throw new IncompatibleSchemaException(); |
| } |
| } |
| |
| /** |
| * Resolve a prior column against the current table schema. Resolves to |
| * a table column, a null column, or throws an exception if the |
| * schemas are incompatible |
| * |
| * @param priorCol a column from the prior schema |
| * @throws IncompatibleSchemaException if the prior column exists in |
| * the current table schema, but with an incompatible type |
| */ |
| |
| private void resolveColumn(ResolvedTuple outputTuple, |
| ResolvedColumn priorCol, TupleMetadata tableSchema) throws IncompatibleSchemaException { |
| int tableColIndex = tableSchema.index(priorCol.name()); |
| if (tableColIndex == -1) { |
| resolveNullColumn(outputTuple, priorCol); |
| return; |
| } |
| MaterializedField tableCol = tableSchema.column(tableColIndex); |
| MaterializedField priorField = priorCol.schema(); |
| if (! tableCol.isPromotableTo(priorField, false)) { |
| throw new IncompatibleSchemaException(); |
| } |
| outputTuple.add( |
| new ResolvedTableColumn(priorCol.name(), priorField, outputTuple, tableColIndex)); |
| rewrittenFields.add(priorField); |
| } |
| |
| /** |
| * A prior schema column does not exist in the present table column schema. |
| * Create a null column with the same type as the prior column, as long as |
| * the prior column was not required. |
| * |
| * @param priorCol the prior column to project to a null column |
| * @throws IncompatibleSchemaException if the prior column was required |
| * and thus cannot be null-filled |
| */ |
| |
| private void resolveNullColumn(ResolvedTuple outputTuple, |
| ResolvedColumn priorCol) throws IncompatibleSchemaException { |
| if (priorCol.schema().getType().getMode() == DataMode.REQUIRED) { |
| throw new IncompatibleSchemaException(); |
| } |
| outputTuple.add(outputTuple.nullBuilder().add(priorCol.name(), |
| priorCol.schema().getType())); |
| } |
| |
| public List<MaterializedField> revisedTableSchema() { return rewrittenFields; } |
| } |