presto-main/src/main/java/io/prestosql/sql/planner/iterative/rule/PushProjectionThroughJoin.java/PushProjectionThroughJoin/pushProjectionThroughJoin
class PushProjectionThroughJoin:    public static Optional<PlanNode> pushProjectionThroughJoin(Metadata metadata, ProjectNode projectNode, Lookup lookup, PlanNodeIdAllocator planNodeIdAllocator)
    {
        if (!projectNode.getAssignments().getExpressions().stream().allMatch(expression -> isDeterministic(expression, metadata))) {
            return Optional.empty();
        }

        PlanNode child = lookup.resolve(projectNode.getSource());
        if (!(child instanceof JoinNode)) {
            return Optional.empty();
        }

        JoinNode joinNode = (JoinNode) child;
        PlanNode leftChild = joinNode.getLeft();
        PlanNode rightChild = joinNode.getRight();

        if (joinNode.getType() != INNER) {
            return Optional.empty();
        }

        Assignments.Builder leftAssignmentsBuilder = Assignments.builder();
        Assignments.Builder rightAssignmentsBuilder = Assignments.builder();
        for (Map.Entry<Symbol, Expression> assignment : projectNode.getAssignments().entrySet()) {
            Expression expression = assignment.getValue();
            Set<Symbol> symbols = extractUnique(expression);
            if (leftChild.getOutputSymbols().containsAll(symbols)) {
                // expression is satisfied with left child symbols
                leftAssignmentsBuilder.put(assignment.getKey(), expression);
            }
            else if (rightChild.getOutputSymbols().containsAll(symbols)) {
                // expression is satisfied with right child symbols
                rightAssignmentsBuilder.put(assignment.getKey(), expression);
            }
            else {
                // expression is using symbols from both join sides
                return Optional.empty();
            }
        }

        // add projections for symbols required by the join itself
        Set<Symbol> joinRequiredSymbols = getJoinRequiredSymbols(joinNode);
        for (Symbol requiredSymbol : joinRequiredSymbols) {
            if (leftChild.getOutputSymbols().contains(requiredSymbol)) {
                leftAssignmentsBuilder.putIdentity(requiredSymbol);
            }
            else {
                checkState(rightChild.getOutputSymbols().contains(requiredSymbol));
                rightAssignmentsBuilder.putIdentity(requiredSymbol);
            }
        }

        Assignments leftAssignments = leftAssignmentsBuilder.build();
        Assignments rightAssignments = rightAssignmentsBuilder.build();
        List<Symbol> outputSymbols = Streams.concat(leftAssignments.getOutputs().stream(), rightAssignments.getOutputs().stream())
                .filter(ImmutableSet.copyOf(projectNode.getOutputSymbols())::contains)
                .collect(toImmutableList());

        return Optional.of(new JoinNode(
                joinNode.getId(),
                joinNode.getType(),
                inlineProjections(
                        new ProjectNode(planNodeIdAllocator.getNextId(), leftChild, leftAssignments),
                        lookup),
                inlineProjections(
                        new ProjectNode(planNodeIdAllocator.getNextId(), rightChild, rightAssignments),
                        lookup),
                joinNode.getCriteria(),
                outputSymbols,
                joinNode.getFilter(),
                joinNode.getLeftHashSymbol(),
                joinNode.getRightHashSymbol(),
                joinNode.getDistributionType(),
                joinNode.isSpillable(),
                joinNode.getDynamicFilters()));
    }