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.optimizer.rules;
  
  import java.util.ArrayList;
  import java.util.List;
  
  import org.apache.asterix.om.base.IACollection;
  import org.apache.asterix.om.base.IACursor;
  import org.apache.asterix.om.base.IAObject;
  import org.apache.asterix.om.constants.AsterixConstantValue;
  import org.apache.asterix.om.functions.BuiltinFunctions;
  import org.apache.commons.lang3.mutable.Mutable;
  import org.apache.commons.lang3.mutable.MutableObject;
  import org.apache.hyracks.algebricks.common.exceptions.AlgebricksException;
  import org.apache.hyracks.algebricks.core.algebra.base.ILogicalExpression;
  import org.apache.hyracks.algebricks.core.algebra.base.ILogicalOperator;
  import org.apache.hyracks.algebricks.core.algebra.base.IOptimizationContext;
  import org.apache.hyracks.algebricks.core.algebra.base.LogicalExpressionTag;
  import org.apache.hyracks.algebricks.core.algebra.base.LogicalOperatorTag;
  import org.apache.hyracks.algebricks.core.algebra.base.LogicalVariable;
  import org.apache.hyracks.algebricks.core.algebra.expressions.AbstractFunctionCallExpression;
  import org.apache.hyracks.algebricks.core.algebra.expressions.ConstantExpression;
  import org.apache.hyracks.algebricks.core.algebra.expressions.IAlgebricksConstantValue;
  import org.apache.hyracks.algebricks.core.algebra.operators.logical.AbstractLogicalOperator;
  import org.apache.hyracks.algebricks.core.algebra.operators.logical.AssignOperator;
  import org.apache.hyracks.algebricks.core.algebra.operators.logical.UnnestOperator;
  import org.apache.hyracks.algebricks.core.rewriter.base.IAlgebraicRewriteRule;
  
  /**
   * This rule removes UNNEST of a constant singleton list and replaces it with ASSIGN as follows:
   * Before plan:
   * <ul>
   * <li>unnest $x <- scan-collection( [ constant ] ) (or {{ constant }})
   * </ul>
   * <p>
   * After plan:
   * <ul>
   * <li>assign $x <- constant
   * </ul>
   *
   * This rule must run after {@link ConstantFoldingRule}
   */
  public final class CancelUnnestSingletonListRule implements IAlgebraicRewriteRule {
      @Override
      public boolean rewritePre(Mutable<ILogicalOperator> opRef, IOptimizationContext context)
              throws AlgebricksException {
          AbstractLogicalOperator op = (AbstractLogicalOperator) opRef.getValue();
          if (op.getOperatorTag() != LogicalOperatorTag.UNNEST) {
              return false;
          }
          UnnestOperator unnest = (UnnestOperator) op;
          if (unnest.getPositionalVariable() != null) {
              return false;
          }
          ILogicalExpression expr = unnest.getExpressionRef().getValue();
          if (expr.getExpressionTag() != LogicalExpressionTag.FUNCTION_CALL) {
              return false;
          }
          if (((AbstractFunctionCallExpression) expr).getFunctionIdentifier() != BuiltinFunctions.SCAN_COLLECTION) {
              return false;
          }
          AbstractFunctionCallExpression callExpr = (AbstractFunctionCallExpression) expr;
          ILogicalExpression argExpr = callExpr.getArguments().get(0).getValue();
          if (argExpr.getExpressionTag() != LogicalExpressionTag.CONSTANT) {
              return false;
          }
          ConstantExpression cExpr = (ConstantExpression) argExpr;
          IAlgebricksConstantValue cValue = cExpr.getValue();
          if (!(cValue instanceof AsterixConstantValue)) {
              return false;
          }
          AsterixConstantValue aValue = (AsterixConstantValue) cValue;
          IAObject value = aValue.getObject();
          if (!value.getType().getTypeTag().isListType()) {
              return false;
          }
          IACollection list = (IACollection) value;
          if (list.size() != 1) {
              return false;
          }
          IACursor cur = list.getCursor();
          if (!cur.next()) {
              return false;
          }
          IAObject item = cur.get();
  
          List<LogicalVariable> assignVars = new ArrayList<>(1);
          assignVars.add(unnest.getVariable());
          List<Mutable<ILogicalExpression>> assignExprs = new ArrayList<>(1);
          ConstantExpression itemExpr = new ConstantExpression(new AsterixConstantValue(item));
          itemExpr.setSourceLocation(cExpr.getSourceLocation());
          assignExprs.add(new MutableObject<>(itemExpr));
          AssignOperator assignOp = new AssignOperator(assignVars, assignExprs);
          assignOp.setSourceLocation(op.getSourceLocation());
          assignOp.getInputs().addAll(op.getInputs());
  
          context.computeAndSetTypeEnvironmentForOperator(assignOp);
          opRef.setValue(assignOp);
          return true;
      }
  
      @Override
      public boolean rewritePost(Mutable<ILogicalOperator> opRef, IOptimizationContext context) {
          return false;
      }
  }
  