Github user zuyu commented on a diff in the pull request:

    https://github.com/apache/incubator-quickstep/pull/237#discussion_r112848590
  
    --- Diff: query_optimizer/rules/ReuseAggregateExpressions.cpp ---
    @@ -0,0 +1,349 @@
    +/**
    + * 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.
    + **/
    +
    +#include "query_optimizer/rules/ReuseAggregateExpressions.hpp"
    +
    +#include <cstddef>
    +#include <list>
    +#include <map>
    +#include <memory>
    +#include <unordered_map>
    +#include <vector>
    +
    +#include "expressions/aggregation/AggregateFunction.hpp"
    +#include "expressions/aggregation/AggregateFunctionFactory.hpp"
    +#include "expressions/aggregation/AggregationID.hpp"
    +#include "query_optimizer/OptimizerContext.hpp"
    +#include "query_optimizer/expressions/AggregateFunction.hpp"
    +#include "query_optimizer/expressions/Alias.hpp"
    +#include "query_optimizer/expressions/AttributeReference.hpp"
    +#include "query_optimizer/expressions/BinaryExpression.hpp"
    +#include "query_optimizer/expressions/ExpressionType.hpp"
    +#include "query_optimizer/expressions/ExpressionUtil.hpp"
    +#include "query_optimizer/expressions/NamedExpression.hpp"
    +#include "query_optimizer/expressions/Scalar.hpp"
    +#include "query_optimizer/physical/Aggregate.hpp"
    +#include "query_optimizer/physical/PatternMatcher.hpp"
    +#include "query_optimizer/physical/Physical.hpp"
    +#include "query_optimizer/physical/PhysicalType.hpp"
    +#include "query_optimizer/physical/Selection.hpp"
    +#include "query_optimizer/physical/TopLevelPlan.hpp"
    +#include "types/operations/binary_operations/BinaryOperation.hpp"
    +#include "types/operations/binary_operations/BinaryOperationFactory.hpp"
    +#include "types/operations/binary_operations/BinaryOperationID.hpp"
    +#include "utility/HashError.hpp"
    +
    +#include "gflags/gflags.h"
    +
    +#include "glog/logging.h"
    +
    +namespace quickstep {
    +namespace optimizer {
    +
    +DEFINE_uint64(reuse_aggregate_group_size_threshold, 1000u,
    +              "The threshold on estimated number of groups for an 
aggregation "
    +              "below which the ReuseAggregateExpressions optimization will 
be "
    +              "performed.");
    +
    +DEFINE_double(reuse_aggregate_ratio_threshold, 0.3,
    +              "The threshold on the ratio of (# of eliminable columns) to "
    +              "(# of original columns) for an aggregation above which the "
    +              "ReuseAggregateExpressions optimization will be performed.");
    +
    +namespace E = ::quickstep::optimizer::expressions;
    +namespace P = ::quickstep::optimizer::physical;
    +
    +void ReuseAggregateExpressions::init(const P::PhysicalPtr &input) {
    +  DCHECK(input->getPhysicalType() == P::PhysicalType::kTopLevelPlan);
    +
    +  // Initialize cost model.
    +  const P::TopLevelPlanPtr top_level_plan =
    +     std::static_pointer_cast<const P::TopLevelPlan>(input);
    +  cost_model_.reset(
    +      new 
cost::StarSchemaSimpleCostModel(top_level_plan->shared_subplans()));
    +}
    +
    +P::PhysicalPtr ReuseAggregateExpressions::applyToNode(
    +    const P::PhysicalPtr &input) {
    +  P::AggregatePtr aggregate;
    +  if (!P::SomeAggregate::MatchesWithConditionalCast(input, &aggregate)) {
    +    return input;
    +  }
    +
    +  const std::vector<E::AliasPtr> &agg_exprs = 
aggregate->aggregate_expressions();
    +
    +  // Maps aggregated expressions to AggregationID + positions.
    +  //
    +  // For example,
    +  // --
    +  //   SELECT SUM(x+1), AVG(x+1), SUM(x+1), COUNT(*), SUM(y) FROM s;
    +  // --
    +  // will generate *agg_expr_info* as
    +  // --
    +  // {
    +  //   x+1: { kSum: [0, 2], kAvg: [1] },
    +  //   y: { kSum: [4] },
    +  // }
    +  // --
    +  // and COUNT(*) will be recorded inside *count_star_info*.
    +  std::unordered_map<E::ScalarPtr,
    +                     std::map<AggregationID, std::vector<std::size_t>>,
    +                     ScalarHash, ScalarEqual> agg_expr_info;
    +  std::list<std::size_t> count_star_info;
    +
    +  for (std::size_t i = 0; i < agg_exprs.size(); ++i) {
    +    DCHECK(agg_exprs[i]->expression()->getExpressionType()
    +               == E::ExpressionType::kAggregateFunction);
    +    const E::AggregateFunctionPtr agg_expr =
    +        std::static_pointer_cast<const E::AggregateFunction>(
    +            agg_exprs[i]->expression());
    +
    +    // Skip DISTINCT aggregations.
    +    if (agg_expr->is_distinct()) {
    +      continue;
    +    }
    +
    +    const AggregationID agg_id = 
agg_expr->getAggregate().getAggregationID();
    +    const std::vector<E::ScalarPtr> &arguments = agg_expr->getArguments();
    +
    +    // Currently we only consider aggregate functions with 0 or 1 argument.
    +    if (agg_id == AggregationID::kCount) {
    +      if (arguments.empty()) {
    +        count_star_info.emplace_front(i);
    +        continue;
    +      } else if (!arguments.front()->getValueType().isNullable()) {
    +        // For COUNT(x) where x is not null, we view it as a more general 
COUNT(*).
    +        count_star_info.emplace_back(i);
    +        continue;
    +      }
    +    }
    +    if (arguments.size() == 1) {
    +      try {
    +        agg_expr_info[arguments.front()][agg_id].emplace_back(i);
    +      } catch (const HashNotSupported &e) {
    +        continue;
    +      }
    +    }
    +  }
    +
    +  // Now for each aggregate expression, figure out whether we can avoid the
    +  // computation by reusing other aggregate expression's result.
    +  std::vector<std::unique_ptr<AggregateReference>> 
agg_refs(agg_exprs.size());
    +
    +  constexpr std::size_t kInvalidRef = static_cast<std::size_t>(-1);
    +  std::size_t count_star_ref;
    +
    +  // Check whether COUNT(*) is available.
    +  if (count_star_info.empty()) {
    +    count_star_ref = kInvalidRef;
    +  } else {
    +    auto it = count_star_info.begin();
    +    count_star_ref = *it;
    +
    +    for (++it; it != count_star_info.end(); ++it) {
    +      agg_refs[*it].reset(new AggregateReference(count_star_ref));
    +    }
    +  }
    +
    +  // Iterate through agg_expr_info to find all transformation 
opportunities,
    +  // and record them into agg_refs.
    +  for (const auto &it : agg_expr_info) {
    +    const auto &ref_map = it.second;
    +
    +    // First, check whether AVG can be reduced to SUM/COUNT.
    +    bool is_avg_processed = false;
    +
    +    const auto avg_it = ref_map.find(AggregationID::kAvg);
    +    if (avg_it != ref_map.end()) {
    +      std::size_t count_ref = kInvalidRef;
    +
    +      // To reduce AVG to SUM/COUNT, we need a COUNT available.
    +      // TODO(jianqiao): We may even add a new COUNT(*) aggregation if it 
is not
    +      // there. E.g. when there are at least two AVG(...) aggregate 
functions.
    +      if (it.first->getValueType().isNullable()) {
    +        const auto count_it = ref_map.find(AggregationID::kCount);
    +        if (count_it != ref_map.end()) {
    +          DCHECK(!count_it->second.empty());
    +          count_ref = count_it->second.front();
    +        }
    +      } else {
    +        count_ref = count_star_ref;
    +      }
    +
    +      if (count_ref != kInvalidRef) {
    +        // It is done if there is an existing SUM. Otherwise we 
strength-reduce
    +        // the current AVG to SUM.
    +        const auto sum_it = ref_map.find(AggregationID::kSum);
    +        const std::size_t sum_ref =
    +            sum_it == ref_map.end() ? kInvalidRef : sum_it->second.front();
    +
    +        for (const std::size_t idx : avg_it->second) {
    +          agg_refs[idx].reset(new AggregateReference(sum_ref, count_ref));
    +        }
    +        is_avg_processed = true;
    +      }
    +    }
    +
    +    // Then, identify duplicate aggregate expressions.
    +    for (const auto &ref_it : ref_map) {
    +      if (ref_it.first == AggregationID::kAvg && is_avg_processed) {
    +        continue;
    +      }
    +      const auto &indices = ref_it.second;
    +      DCHECK(!indices.empty());
    +      const std::size_t ref = indices.front();
    +      for (std::size_t i = 1; i < indices.size(); ++i) {
    +        agg_refs[indices[i]].reset(new AggregateReference(ref));
    +      }
    +    }
    +  }
    +
    +  // Count the number of eliminable aggregate expressions.
    +  std::size_t num_eliminable = 0;
    +  for (const auto &agg_ref : agg_refs) {
    +    if (agg_ref != nullptr) {
    +      ++num_eliminable;
    +    }
    +  }
    +
    +  if (num_eliminable == 0) {
    +    return input;
    +  }
    +
    +  // Now we need to make a decision since it is not always benefitial to 
perform
    +  // the transformation. Currently we employ a simple heuristic that if 
either
    +  // (1) The estimated number of groups for this Aggregate node is smaller 
than
    +  //     the specified FLAGS_reuse_aggregate_group_size_threshold.
    +  // or
    +  // (2) The ratio of (# of eliminable columns) to (# of original columns) 
is
    +  //     greater than the specified FLAGS_reuse_aggregate_ratio_threshold.
    +  // then we perform the transformation.
    +  const bool is_group_size_condition_satisfied =
    +      cost_model_->estimateNumGroupsForAggregate(aggregate)
    +          < FLAGS_reuse_aggregate_group_size_threshold;
    +  const bool is_ratio_condition_satisfied =
    +      static_cast<double>(num_eliminable) / agg_exprs.size()
    +          > FLAGS_reuse_aggregate_ratio_threshold;
    +
    +  if (!is_group_size_condition_satisfied && !is_ratio_condition_satisfied) 
{
    +    return input;
    +  }
    +
    +  // Now we transform the original Aggregate to a reduced Aggregate + a 
wrapping
    +  // Selection.
    +
    +  // Aggregate expressions for the new Aggregate.
    +  std::vector<E::AliasPtr> new_agg_exprs;
    +
    +  // Project expressions for the new Selection.
    +  std::vector<E::NamedExpressionPtr> new_select_exprs;
    +
    +  for (const auto &grouping_expr : aggregate->grouping_expressions()) {
    +    new_select_exprs.emplace_back(E::ToRef(grouping_expr));
    +  }
    +
    +  const std::vector<E::AttributeReferencePtr> agg_attrs = 
E::ToRefVector(agg_exprs);
    +
    +  for (std::size_t i = 0; i < agg_refs.size(); ++i) {
    +    const auto &agg_ref = agg_refs[i];
    +    const E::AliasPtr &agg_expr = agg_exprs[i];
    +
    +    if (agg_ref == nullptr) {
    +      // Case 1: this aggregate expression can not be eliminated.
    +      new_agg_exprs.emplace_back(agg_expr);
    +      new_select_exprs.emplace_back(
    +          E::AttributeReference::Create(agg_expr->id(),
    +                                        agg_expr->attribute_name(),
    +                                        agg_expr->attribute_alias(),
    +                                        agg_expr->relation_name(),
    +                                        agg_expr->getValueType(),
    +                                        
E::AttributeReferenceScope::kLocal));
    +    } else {
    +      switch (agg_ref->kind) {
    +        // Case 2.1: this aggregate expression can be eliminated.
    +        case AggregateReference::kDirect: {
    +          new_select_exprs.emplace_back(
    +              E::Alias::Create(agg_expr->id(),
    +                               agg_attrs[agg_ref->first_ref],
    +                               agg_expr->attribute_name(),
    +                               agg_expr->attribute_alias(),
    +                               agg_expr->relation_name()));
    +          break;
    +        }
    +        // Case 2.2: this aggregate expression is an AVG.
    +        case AggregateReference::kAvg: {
    +          E::AttributeReferencePtr sum_attr;
    +          if (agg_ref->first_ref == kInvalidRef) {
    +            // Case 2.2.1: If there is no existing SUM, we need to convert 
this
    +            //             AVG to SUM.
    +            const E::AggregateFunctionPtr avg_expr =
    +                std::static_pointer_cast<const 
E::AggregateFunction>(agg_expr->expression());
    +
    +            const AggregateFunction &sum_func =
    +                AggregateFunctionFactory::Get(AggregationID::kSum);
    +            const E::AggregateFunctionPtr sum_expr =
    +                E::AggregateFunction::Create(sum_func,
    +                                             avg_expr->getArguments(),
    +                                             
avg_expr->is_vector_aggregate(),
    +                                             avg_expr->is_distinct());
    +            new_agg_exprs.emplace_back(
    +                E::Alias::Create(optimizer_context_->nextExprId(),
    +                                 sum_expr,
    +                                 agg_expr->attribute_name(),
    +                                 agg_expr->attribute_alias(),
    +                                 agg_expr->relation_name()));
    +
    +            sum_attr = E::ToRef(new_agg_exprs.back());
    +          } else {
    +            // Case 2.2.2: If there is a SUM somewhere, we just eliminate 
this
    +            //             AVG and use the result from that SUM.
    +            sum_attr = agg_attrs[agg_ref->first_ref];
    +          }
    +
    +          // Obtain AVG by evaluating SUM/COUNT in Selection.
    +          const BinaryOperation &divide_op =
    +              
BinaryOperationFactory::GetBinaryOperation(BinaryOperationID::kDivide);
    +          const E::BinaryExpressionPtr avg_expr =
    +              E::BinaryExpression::Create(divide_op,
    +                                          sum_attr,
    +                                          agg_attrs[agg_ref->second_ref]);
    +          new_select_exprs.emplace_back(
    +              E::Alias::Create(agg_expr->id(),
    +                               avg_expr,
    +                               agg_expr->attribute_name(),
    +                               agg_expr->attribute_alias(),
    +                               agg_expr->relation_name()));
    +        }
    +      }
    --- End diff --
    
    To be safe, please add a `break;` here.


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