Examples with SelectOperator org.apache.hyracks.algebricks.core.algebra.operators.logical.SelectOperator used on opensource projects

Example 26 with SelectOperator

use of org.apache.hyracks.algebricks.core.algebra.operators.logical.SelectOperator in project asterixdb by apache.

the class InlineSubplanInputForNestedTupleSourceRule method applySpecialFlattening.

private Pair<Boolean, LinkedHashMap<LogicalVariable, LogicalVariable>> applySpecialFlattening(Mutable<ILogicalOperator> opRef, IOptimizationContext context) throws AlgebricksException {
    SubplanOperator subplanOp = (SubplanOperator) opRef.getValue();
    Mutable<ILogicalOperator> inputOpRef = subplanOp.getInputs().get(0);
    LinkedHashMap<LogicalVariable, LogicalVariable> replacedVarMap = new LinkedHashMap<>();
    // Recursively applies this rule to the nested plan of the subplan operator,
    // for the case where there are nested subplan operators within {@code subplanOp}.
    Pair<Boolean, LinkedHashMap<LogicalVariable, LogicalVariable>> result = rewriteSubplanOperator(subplanOp.getNestedPlans().get(0).getRoots().get(0), context);
    ILogicalOperator inputOpBackup = inputOpRef.getValue();
    // Gets live variables and covering variables from the subplan's input operator.
    Pair<ILogicalOperator, Set<LogicalVariable>> primaryOpAndVars = EquivalenceClassUtils.findOrCreatePrimaryKeyOpAndVariables(inputOpBackup, false, context);
    ILogicalOperator inputOp = primaryOpAndVars.first;
    Set<LogicalVariable> primaryKeyVars = primaryOpAndVars.second;
    inputOpRef.setValue(inputOp);
    Set<LogicalVariable> liveVars = new HashSet<>();
    VariableUtilities.getLiveVariables(inputOp, liveVars);
    Pair<Set<LogicalVariable>, Mutable<ILogicalOperator>> notNullVarsAndTopJoinRef = SubplanFlatteningUtil.inlineLeftNtsInSubplanJoin(subplanOp, context);
    if (notNullVarsAndTopJoinRef.first == null) {
        inputOpRef.setValue(inputOpBackup);
        return new Pair<>(false, replacedVarMap);
    }
    Set<LogicalVariable> notNullVars = notNullVarsAndTopJoinRef.first;
    Mutable<ILogicalOperator> topJoinRef = notNullVarsAndTopJoinRef.second;
    // Creates a group-by operator.
    List<Pair<LogicalVariable, Mutable<ILogicalExpression>>> groupByList = new ArrayList<Pair<LogicalVariable, Mutable<ILogicalExpression>>>();
    List<Pair<LogicalVariable, Mutable<ILogicalExpression>>> groupByDecorList = new ArrayList<Pair<LogicalVariable, Mutable<ILogicalExpression>>>();
    GroupByOperator groupbyOp = new GroupByOperator(groupByList, groupByDecorList, subplanOp.getNestedPlans());
    for (LogicalVariable coverVar : primaryKeyVars) {
        LogicalVariable newVar = context.newVar();
        groupByList.add(new Pair<>(newVar, new MutableObject<>(new VariableReferenceExpression(coverVar))));
        // Adds variables for replacements in ancestors.
        replacedVarMap.put(coverVar, newVar);
    }
    for (LogicalVariable liveVar : liveVars) {
        if (primaryKeyVars.contains(liveVar)) {
            continue;
        }
        groupByDecorList.add(new Pair<>(null, new MutableObject<>(new VariableReferenceExpression(liveVar))));
    }
    groupbyOp.getInputs().add(new MutableObject<>(topJoinRef.getValue()));
    if (!notNullVars.isEmpty()) {
        // Adds a select operator into the nested plan for group-by to remove tuples with NULL on {@code assignVar}, i.e.,
        // subplan input tuples that are filtered out within a subplan.
        List<Mutable<ILogicalExpression>> nullCheckExprRefs = new ArrayList<>();
        for (LogicalVariable notNullVar : notNullVars) {
            Mutable<ILogicalExpression> filterVarExpr = new MutableObject<>(new VariableReferenceExpression(notNullVar));
            List<Mutable<ILogicalExpression>> args = new ArrayList<>();
            args.add(filterVarExpr);
            List<Mutable<ILogicalExpression>> argsForNotFunction = new ArrayList<>();
            argsForNotFunction.add(new MutableObject<>(new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(BuiltinFunctions.IS_MISSING), args)));
            nullCheckExprRefs.add(new MutableObject<>(new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(BuiltinFunctions.NOT), argsForNotFunction)));
        }
        Mutable<ILogicalExpression> selectExprRef = nullCheckExprRefs.size() > 1 ? new MutableObject<>(new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(BuiltinFunctions.AND), nullCheckExprRefs)) : nullCheckExprRefs.get(0);
        SelectOperator selectOp = new SelectOperator(selectExprRef, false, null);
        topJoinRef.setValue(selectOp);
        selectOp.getInputs().add(new MutableObject<>(new NestedTupleSourceOperator(new MutableObject<>(groupbyOp))));
    } else {
        // The original join operator in the Subplan is a left-outer join.
        // Therefore, no null-check variable is injected and no SelectOperator needs to be added.
        topJoinRef.setValue(new NestedTupleSourceOperator(new MutableObject<>(groupbyOp)));
    }
    opRef.setValue(groupbyOp);
    OperatorManipulationUtil.computeTypeEnvironmentBottomUp(groupbyOp, context);
    VariableUtilities.substituteVariables(groupbyOp, result.second, context);
    replacedVarMap.putAll(result.second);
    return new Pair<>(true, replacedVarMap);
}

Example 27 with SelectOperator

use of org.apache.hyracks.algebricks.core.algebra.operators.logical.SelectOperator in project asterixdb by apache.

the class TranslateIntervalExpressionRule method rewritePost.

@Override
public boolean rewritePost(Mutable<ILogicalOperator> opRef, IOptimizationContext context) throws AlgebricksException {
    AbstractLogicalOperator op = (AbstractLogicalOperator) opRef.getValue();
    if (op.getOperatorTag() != LogicalOperatorTag.SELECT) {
        return false;
    }
    SelectOperator selectOp = (SelectOperator) op;
    Mutable<ILogicalExpression> exprRef = selectOp.getCondition();
    ILogicalExpression expr = exprRef.getValue();
    if (expr.getExpressionTag() != LogicalExpressionTag.FUNCTION_CALL) {
        return false;
    }
    AbstractFunctionCallExpression funcExpr = (AbstractFunctionCallExpression) expr;
    if (funcExpr.getArguments().size() != 2) {
        return false;
    }
    if (!hasTranslatableInterval(funcExpr)) {
        return false;
    }
    return translateIntervalExpression(exprRef, funcExpr);
}

Example 28 with SelectOperator

use of org.apache.hyracks.algebricks.core.algebra.operators.logical.SelectOperator in project asterixdb by apache.

the class InlineSubplanInputForNestedTupleSourceRule method applyGeneralFlattening.

private Pair<Boolean, LinkedHashMap<LogicalVariable, LogicalVariable>> applyGeneralFlattening(Mutable<ILogicalOperator> opRef, IOptimizationContext context) throws AlgebricksException {
    SubplanOperator subplanOp = (SubplanOperator) opRef.getValue();
    if (!SubplanFlatteningUtil.containsOperators(subplanOp, ImmutableSet.of(LogicalOperatorTag.DATASOURCESCAN, LogicalOperatorTag.INNERJOIN, // We don't have nested runtime for union-all and distinct hence we have to include them here.
    LogicalOperatorTag.LEFTOUTERJOIN, LogicalOperatorTag.UNIONALL, LogicalOperatorTag.DISTINCT))) {
        return new Pair<>(false, new LinkedHashMap<>());
    }
    Mutable<ILogicalOperator> inputOpRef = subplanOp.getInputs().get(0);
    ILogicalOperator inputOpBackup = inputOpRef.getValue();
    // Creates parameters for the left outer join operator.
    Pair<ILogicalOperator, Set<LogicalVariable>> primaryOpAndVars = EquivalenceClassUtils.findOrCreatePrimaryKeyOpAndVariables(inputOpBackup, true, context);
    ILogicalOperator inputOp = primaryOpAndVars.first;
    Set<LogicalVariable> primaryKeyVars = primaryOpAndVars.second;
    inputOpRef.setValue(inputOp);
    Set<LogicalVariable> inputLiveVars = new HashSet<>();
    VariableUtilities.getLiveVariables(inputOp, inputLiveVars);
    Pair<Map<LogicalVariable, LogicalVariable>, List<Pair<IOrder, Mutable<ILogicalExpression>>>> varMapAndOrderExprs = SubplanFlatteningUtil.inlineAllNestedTupleSource(subplanOp, context);
    Map<LogicalVariable, LogicalVariable> varMap = varMapAndOrderExprs.first;
    if (varMap == null) {
        inputOpRef.setValue(inputOpBackup);
        return new Pair<>(false, new LinkedHashMap<>());
    }
    Mutable<ILogicalOperator> lowestAggregateRefInSubplan = SubplanFlatteningUtil.findLowestAggregate(subplanOp.getNestedPlans().get(0).getRoots().get(0));
    Mutable<ILogicalOperator> rightInputOpRef = lowestAggregateRefInSubplan.getValue().getInputs().get(0);
    ILogicalOperator rightInputOp = rightInputOpRef.getValue();
    // Creates a variable to indicate whether a left input tuple is killed in the plan rooted at rightInputOp.
    LogicalVariable assignVar = context.newVar();
    ILogicalOperator assignOp = new AssignOperator(assignVar, new MutableObject<>(ConstantExpression.TRUE));
    assignOp.getInputs().add(rightInputOpRef);
    context.computeAndSetTypeEnvironmentForOperator(assignOp);
    rightInputOpRef = new MutableObject<>(assignOp);
    // Constructs the join predicate for the leftOuter join.
    List<Mutable<ILogicalExpression>> joinPredicates = new ArrayList<>();
    for (LogicalVariable liveVar : primaryKeyVars) {
        List<Mutable<ILogicalExpression>> arguments = new ArrayList<>();
        arguments.add(new MutableObject<>(new VariableReferenceExpression(liveVar)));
        LogicalVariable rightVar = varMap.get(liveVar);
        arguments.add(new MutableObject<>(new VariableReferenceExpression(rightVar)));
        ILogicalExpression expr = new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(AlgebricksBuiltinFunctions.EQ), arguments);
        joinPredicates.add(new MutableObject<>(expr));
    }
    ILogicalExpression joinExpr = joinPredicates.size() > 1 ? new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(AlgebricksBuiltinFunctions.AND), joinPredicates) : joinPredicates.size() > 0 ? joinPredicates.get(0).getValue() : ConstantExpression.TRUE;
    LeftOuterJoinOperator leftOuterJoinOp = new LeftOuterJoinOperator(new MutableObject<>(joinExpr), inputOpRef, rightInputOpRef);
    OperatorManipulationUtil.computeTypeEnvironmentBottomUp(rightInputOp, context);
    context.computeAndSetTypeEnvironmentForOperator(leftOuterJoinOp);
    // Creates group-by operator.
    List<Pair<LogicalVariable, Mutable<ILogicalExpression>>> groupByList = new ArrayList<Pair<LogicalVariable, Mutable<ILogicalExpression>>>();
    List<Pair<LogicalVariable, Mutable<ILogicalExpression>>> groupByDecorList = new ArrayList<Pair<LogicalVariable, Mutable<ILogicalExpression>>>();
    List<ILogicalPlan> nestedPlans = new ArrayList<>();
    GroupByOperator groupbyOp = new GroupByOperator(groupByList, groupByDecorList, nestedPlans);
    LinkedHashMap<LogicalVariable, LogicalVariable> replacedVarMap = new LinkedHashMap<>();
    for (LogicalVariable liveVar : primaryKeyVars) {
        LogicalVariable newVar = context.newVar();
        groupByList.add(new Pair<>(newVar, new MutableObject<>(new VariableReferenceExpression(liveVar))));
        // Adds variables for replacements in ancestors.
        replacedVarMap.put(liveVar, newVar);
    }
    for (LogicalVariable liveVar : inputLiveVars) {
        if (primaryKeyVars.contains(liveVar)) {
            continue;
        }
        groupByDecorList.add(new Pair<>(null, new MutableObject<>(new VariableReferenceExpression(liveVar))));
    }
    // Sets up the nested plan for the groupby operator.
    Mutable<ILogicalOperator> aggOpRef = subplanOp.getNestedPlans().get(0).getRoots().get(0);
    // Clears the input of the lowest aggregate.
    lowestAggregateRefInSubplan.getValue().getInputs().clear();
    Mutable<ILogicalOperator> currentOpRef = lowestAggregateRefInSubplan;
    // Adds an optional order operator.
    List<Pair<IOrder, Mutable<ILogicalExpression>>> orderExprs = varMapAndOrderExprs.second;
    if (!orderExprs.isEmpty()) {
        OrderOperator orderOp = new OrderOperator(orderExprs);
        currentOpRef = new MutableObject<>(orderOp);
        lowestAggregateRefInSubplan.getValue().getInputs().add(currentOpRef);
    }
    // Adds a select operator into the nested plan for group-by to remove tuples with NULL on {@code assignVar}, i.e.,
    // subplan input tuples that are filtered out within a subplan.
    Mutable<ILogicalExpression> filterVarExpr = new MutableObject<>(new VariableReferenceExpression(assignVar));
    List<Mutable<ILogicalExpression>> args = new ArrayList<>();
    args.add(filterVarExpr);
    List<Mutable<ILogicalExpression>> argsForNotFunction = new ArrayList<>();
    argsForNotFunction.add(new MutableObject<>(new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(BuiltinFunctions.IS_MISSING), args)));
    SelectOperator selectOp = new SelectOperator(new MutableObject<>(new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(BuiltinFunctions.NOT), argsForNotFunction)), false, null);
    currentOpRef.getValue().getInputs().add(new MutableObject<>(selectOp));
    selectOp.getInputs().add(new MutableObject<>(new NestedTupleSourceOperator(new MutableObject<>(groupbyOp))));
    List<Mutable<ILogicalOperator>> nestedRoots = new ArrayList<>();
    nestedRoots.add(aggOpRef);
    nestedPlans.add(new ALogicalPlanImpl(nestedRoots));
    groupbyOp.getInputs().add(new MutableObject<>(leftOuterJoinOp));
    // Replaces subplan with the group-by operator.
    opRef.setValue(groupbyOp);
    OperatorManipulationUtil.computeTypeEnvironmentBottomUp(groupbyOp, context);
    // Recursively applys this rule to the nested plan of the subplan operator,
    // for the case where there are nested subplan operators within {@code subplanOp}.
    Pair<Boolean, LinkedHashMap<LogicalVariable, LogicalVariable>> result = rewriteSubplanOperator(rightInputOpRef, context);
    VariableUtilities.substituteVariables(leftOuterJoinOp, result.second, context);
    VariableUtilities.substituteVariables(groupbyOp, result.second, context);
    // No var mapping from the right input operator should be populated up.
    return new Pair<>(true, replacedVarMap);
}

Example 29 with SelectOperator

use of org.apache.hyracks.algebricks.core.algebra.operators.logical.SelectOperator in project asterixdb by apache.

the class LangExpressionToPlanTranslator method visit.

@Override
public Pair<ILogicalOperator, LogicalVariable> visit(QuantifiedExpression qe, Mutable<ILogicalOperator> tupSource) throws CompilationException {
    Mutable<ILogicalOperator> topOp = tupSource;
    ILogicalOperator firstOp = null;
    Mutable<ILogicalOperator> lastOp = null;
    for (QuantifiedPair qt : qe.getQuantifiedList()) {
        Pair<ILogicalExpression, Mutable<ILogicalOperator>> eo1 = langExprToAlgExpression(qt.getExpr(), topOp);
        topOp = eo1.second;
        LogicalVariable uVar = context.newVarFromExpression(qt.getVarExpr());
        ILogicalOperator u = new UnnestOperator(uVar, new MutableObject<>(makeUnnestExpression(eo1.first)));
        if (firstOp == null) {
            firstOp = u;
        }
        if (lastOp != null) {
            u.getInputs().add(lastOp);
        }
        lastOp = new MutableObject<>(u);
    }
    // We make all the unnest correspond. to quantif. vars. sit on top
    // in the hope of enabling joins & other optimiz.
    firstOp.getInputs().add(topOp);
    topOp = lastOp;
    Pair<ILogicalExpression, Mutable<ILogicalOperator>> eo2 = langExprToAlgExpression(qe.getSatisfiesExpr(), topOp);
    AggregateFunctionCallExpression fAgg;
    SelectOperator s;
    if (qe.getQuantifier() == Quantifier.SOME) {
        s = new SelectOperator(new MutableObject<>(eo2.first), false, null);
        s.getInputs().add(eo2.second);
        fAgg = BuiltinFunctions.makeAggregateFunctionExpression(BuiltinFunctions.NON_EMPTY_STREAM, new ArrayList<>());
    } else {
        // EVERY
        List<Mutable<ILogicalExpression>> satExprList = new ArrayList<>(1);
        satExprList.add(new MutableObject<>(eo2.first));
        s = new SelectOperator(new MutableObject<>(new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(AlgebricksBuiltinFunctions.NOT), satExprList)), false, null);
        s.getInputs().add(eo2.second);
        fAgg = BuiltinFunctions.makeAggregateFunctionExpression(BuiltinFunctions.EMPTY_STREAM, new ArrayList<>());
    }
    LogicalVariable qeVar = context.newVar();
    AggregateOperator a = new AggregateOperator(mkSingletonArrayList(qeVar), (List) mkSingletonArrayList(new MutableObject<>(fAgg)));
    a.getInputs().add(new MutableObject<>(s));
    return new Pair<>(a, qeVar);
}

Example 30 with SelectOperator

use of org.apache.hyracks.algebricks.core.algebra.operators.logical.SelectOperator in project asterixdb by apache.

the class SqlppExpressionToPlanTranslator method visit.

@Override
public Pair<ILogicalOperator, LogicalVariable> visit(JoinClause joinClause, Mutable<ILogicalOperator> inputRef) throws CompilationException {
    Mutable<ILogicalOperator> leftInputRef = uncorrelatedLeftBranchStack.pop();
    if (joinClause.getJoinType() == JoinType.INNER) {
        Pair<ILogicalOperator, LogicalVariable> rightBranch = generateUnnestForBinaryCorrelateRightBranch(joinClause, inputRef, true);
        // A join operator with condition TRUE.
        AbstractBinaryJoinOperator joinOperator = new InnerJoinOperator(new MutableObject<ILogicalExpression>(ConstantExpression.TRUE), leftInputRef, new MutableObject<ILogicalOperator>(rightBranch.first));
        Mutable<ILogicalOperator> joinOpRef = new MutableObject<>(joinOperator);
        // Add an additional filter operator.
        Pair<ILogicalExpression, Mutable<ILogicalOperator>> conditionExprOpPair = langExprToAlgExpression(joinClause.getConditionExpression(), joinOpRef);
        SelectOperator filter = new SelectOperator(new MutableObject<ILogicalExpression>(conditionExprOpPair.first), false, null);
        filter.getInputs().add(conditionExprOpPair.second);
        return new Pair<>(filter, rightBranch.second);
    } else {
        // Creates a subplan operator.
        SubplanOperator subplanOp = new SubplanOperator();
        Mutable<ILogicalOperator> ntsRef = new MutableObject<>(new NestedTupleSourceOperator(new MutableObject<ILogicalOperator>(subplanOp)));
        subplanOp.getInputs().add(leftInputRef);
        // Enters the translation for a subplan.
        context.enterSubplan();
        // Adds an unnest operator to unnest to right expression.
        Pair<ILogicalOperator, LogicalVariable> rightBranch = generateUnnestForBinaryCorrelateRightBranch(joinClause, ntsRef, true);
        AbstractUnnestNonMapOperator rightUnnestOp = (AbstractUnnestNonMapOperator) rightBranch.first;
        // Adds an additional filter operator for the join condition.
        Pair<ILogicalExpression, Mutable<ILogicalOperator>> conditionExprOpPair = langExprToAlgExpression(joinClause.getConditionExpression(), new MutableObject<ILogicalOperator>(rightUnnestOp));
        SelectOperator filter = new SelectOperator(new MutableObject<ILogicalExpression>(conditionExprOpPair.first), false, null);
        filter.getInputs().add(conditionExprOpPair.second);
        ILogicalOperator currentTopOp = filter;
        LogicalVariable varToListify;
        boolean hasRightPosVar = rightUnnestOp.getPositionalVariable() != null;
        if (hasRightPosVar) {
            // Creates record to get correlation between the two aggregate variables.
            ScalarFunctionCallExpression recordCreationFunc = new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(BuiltinFunctions.CLOSED_RECORD_CONSTRUCTOR), // Field name for the listified right unnest var.
            new MutableObject<ILogicalExpression>(new ConstantExpression(new AsterixConstantValue(new AString("unnestvar")))), // The listified right unnest var
            new MutableObject<ILogicalExpression>(new VariableReferenceExpression(rightUnnestOp.getVariable())), // Field name for the listified right unnest positional var.
            new MutableObject<ILogicalExpression>(new ConstantExpression(new AsterixConstantValue(new AString("posvar")))), // The listified right unnest positional var.
            new MutableObject<ILogicalExpression>(new VariableReferenceExpression(rightUnnestOp.getPositionalVariable())));
            // Assigns the record constructor function to a record variable.
            LogicalVariable recordVar = context.newVar();
            AssignOperator assignOp = new AssignOperator(recordVar, new MutableObject<ILogicalExpression>(recordCreationFunc));
            assignOp.getInputs().add(new MutableObject<ILogicalOperator>(currentTopOp));
            // Sets currentTopOp and varToListify for later usages.
            currentTopOp = assignOp;
            varToListify = recordVar;
        } else {
            varToListify = rightUnnestOp.getVariable();
        }
        // Adds an aggregate operator to listfy unnest variables.
        AggregateFunctionCallExpression fListify = BuiltinFunctions.makeAggregateFunctionExpression(BuiltinFunctions.LISTIFY, mkSingletonArrayList(new MutableObject<ILogicalExpression>(new VariableReferenceExpression(varToListify))));
        LogicalVariable aggVar = context.newSubplanOutputVar();
        AggregateOperator aggOp = new AggregateOperator(mkSingletonArrayList(aggVar), mkSingletonArrayList(new MutableObject<ILogicalExpression>(fListify)));
        aggOp.getInputs().add(new MutableObject<ILogicalOperator>(currentTopOp));
        // Exits the translation of a subplan.
        context.exitSubplan();
        // Sets the nested subplan of the subplan operator.
        ILogicalPlan subplan = new ALogicalPlanImpl(new MutableObject<ILogicalOperator>(aggOp));
        subplanOp.getNestedPlans().add(subplan);
        // Outer unnest the aggregated var from the subplan.
        LogicalVariable outerUnnestVar = context.newVar();
        LeftOuterUnnestOperator outerUnnestOp = new LeftOuterUnnestOperator(outerUnnestVar, new MutableObject<ILogicalExpression>(makeUnnestExpression(new VariableReferenceExpression(aggVar))));
        outerUnnestOp.getInputs().add(new MutableObject<ILogicalOperator>(subplanOp));
        currentTopOp = outerUnnestOp;
        if (hasRightPosVar) {
            ScalarFunctionCallExpression fieldAccessForRightUnnestVar = new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(BuiltinFunctions.FIELD_ACCESS_BY_INDEX), new MutableObject<ILogicalExpression>(new VariableReferenceExpression(outerUnnestVar)), new MutableObject<ILogicalExpression>(new ConstantExpression(new AsterixConstantValue(new AInt32(0)))));
            ScalarFunctionCallExpression fieldAccessForRightPosVar = new ScalarFunctionCallExpression(FunctionUtil.getFunctionInfo(BuiltinFunctions.FIELD_ACCESS_BY_INDEX), new MutableObject<ILogicalExpression>(new VariableReferenceExpression(outerUnnestVar)), new MutableObject<ILogicalExpression>(new ConstantExpression(new AsterixConstantValue(new AInt32(1)))));
            // Creates variables for assign.
            LogicalVariable rightUnnestVar = context.newVar();
            LogicalVariable rightPosVar = context.newVar();
            // Relate the assigned variables to the variable expression in AST.
            context.setVar(joinClause.getRightVariable(), rightUnnestVar);
            context.setVar(joinClause.getPositionalVariable(), rightPosVar);
            // Varaibles to assign.
            List<LogicalVariable> assignVars = new ArrayList<>();
            assignVars.add(rightUnnestVar);
            assignVars.add(rightPosVar);
            // Expressions for assign.
            List<Mutable<ILogicalExpression>> assignExprs = new ArrayList<>();
            assignExprs.add(new MutableObject<ILogicalExpression>(fieldAccessForRightUnnestVar));
            assignExprs.add(new MutableObject<ILogicalExpression>(fieldAccessForRightPosVar));
            // Creates the assign operator.
            AssignOperator assignOp = new AssignOperator(assignVars, assignExprs);
            assignOp.getInputs().add(new MutableObject<ILogicalOperator>(currentTopOp));
            currentTopOp = assignOp;
        } else {
            context.setVar(joinClause.getRightVariable(), outerUnnestVar);
        }
        return new Pair<>(currentTopOp, null);
    }
}