content: /*
   * 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.asterix.lang.sqlpp.rewrites.visitor;
  
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.function.BiFunction;
  
  import org.apache.asterix.common.exceptions.CompilationException;
  import org.apache.asterix.lang.common.base.Expression;
  import org.apache.asterix.lang.common.base.ILangExpression;
  import org.apache.asterix.lang.common.clause.LimitClause;
  import org.apache.asterix.lang.common.clause.OrderbyClause;
  import org.apache.asterix.lang.common.expression.VariableExpr;
  import org.apache.asterix.lang.common.rewrites.LangRewritingContext;
  import org.apache.asterix.lang.sqlpp.clause.FromClause;
  import org.apache.asterix.lang.sqlpp.clause.FromTerm;
  import org.apache.asterix.lang.sqlpp.clause.SelectBlock;
  import org.apache.asterix.lang.sqlpp.clause.SelectClause;
  import org.apache.asterix.lang.sqlpp.clause.SelectElement;
  import org.apache.asterix.lang.sqlpp.clause.SelectSetOperation;
  import org.apache.asterix.lang.sqlpp.expression.SelectExpression;
  import org.apache.asterix.lang.sqlpp.struct.SetOperationInput;
  import org.apache.asterix.lang.sqlpp.visitor.base.AbstractSqlppExpressionScopingVisitor;
  import org.apache.hyracks.api.exceptions.SourceLocation;
  
  /**
   * This visitor rewrites set operation queries with order by and limit into
   * a nested query where the set operation part is a subquery in the from clause.
   * In this way, there is no special variable scoping mechanism that is needed
   * for order by and limit clauses after the set operation.
   */
  /*
  For example, the following query
  
  SELECT ... FROM ...
  UNION ALL
  SELECT ... FROM ...
  ORDER BY foo
  Limit 5;
  
  is rewritten into the following form:
  
  SELECT VALUE v
  FROM (
     SELECT ... FROM ...
     UNION ALL
     SELECT ... FROM ...
   ) AS v
  ORDER BY foo
  LIMIT 5;
  */
  public class SetOperationVisitor extends AbstractSqlppExpressionScopingVisitor {
  
      public SetOperationVisitor(LangRewritingContext context) {
          super(context);
      }
  
      @Override
      public Expression visit(SelectExpression selectExpression, ILangExpression arg) throws CompilationException {
          // Recursively visit nested select expressions.
          SelectSetOperation selectSetOperation = selectExpression.getSelectSetOperation();
          if (!selectSetOperation.hasRightInputs() || !(selectExpression.hasOrderby() || selectExpression.hasLimit())) {
              return super.visit(selectExpression, arg);
          }
          OrderbyClause orderBy = selectExpression.getOrderbyClause();
          LimitClause limit = selectExpression.getLimitClause();
  
          SourceLocation sourceLoc = selectExpression.getSourceLocation();
  
          // Wraps the set operation part with a subquery.
          SelectExpression nestedSelectExpression = new SelectExpression(null, selectSetOperation, null, null, true);
          nestedSelectExpression.setSourceLocation(sourceLoc);
          SelectBlock selectBlock = createSelectBlock(nestedSelectExpression, false, null, null, context);
          SelectSetOperation newSelectSetOperation =
                  new SelectSetOperation(new SetOperationInput(selectBlock, null), null);
          newSelectSetOperation.setSourceLocation(sourceLoc);
          // Puts together the generated select-from-where query and order by/limit.
          SelectExpression newSelectExpression = new SelectExpression(selectExpression.getLetList(),
                  newSelectSetOperation, orderBy, limit, selectExpression.isSubquery());
          newSelectExpression.setSourceLocation(sourceLoc);
          newSelectExpression.addHints(selectExpression.getHints());
          return super.visit(newSelectExpression, arg);
      }
  
      static <P> SelectBlock createSelectBlock(Expression inputExpr, boolean distinct,
              BiFunction<? super VariableExpr, P, Expression> valueExprTransformer, P valueExprTransformerArg,
              LangRewritingContext context) {
          SourceLocation sourceLoc = inputExpr.getSourceLocation();
          VariableExpr newBindingVar = new VariableExpr(context.newVariable()); // Binding variable for the subquery.
          newBindingVar.setSourceLocation(sourceLoc);
          FromTerm newFromTerm = new FromTerm(inputExpr, newBindingVar, null, null);
          newFromTerm.setSourceLocation(sourceLoc);
          FromClause newFromClause = new FromClause(new ArrayList<>(Collections.singletonList(newFromTerm)));
          newFromClause.setSourceLocation(sourceLoc);
          Expression valueExpr = valueExprTransformer != null
                  ? valueExprTransformer.apply(newBindingVar, valueExprTransformerArg) : newBindingVar;
          SelectClause selectClause = new SelectClause(new SelectElement(valueExpr), null, distinct);
          selectClause.setSourceLocation(sourceLoc);
          SelectBlock selectBlock = new SelectBlock(selectClause, newFromClause, null, null, null);
          selectBlock.setSourceLocation(sourceLoc);
          return selectBlock;
      }
  }
  