ClickHouse/dbms/include/DB/Storages/MergeTree/MergeTreeWhereOptimizer.h

312 lines
10 KiB
C++

#pragma once
#include <DB/Storages/MergeTree/MergeTreeData.h>
#include <DB/Parsers/ASTSelectQuery.h>
#include <DB/Parsers/ASTFunction.h>
#include <DB/Parsers/ASTIdentifier.h>
#include <DB/Parsers/ASTLiteral.h>
#include <DB/Parsers/ASTExpressionList.h>
#include <DB/Parsers/ASTSubquery.h>
#include <DB/Parsers/formatAST.h>
#include <DB/Common/escapeForFileName.h>
#include <statdaemons/ext/memory.hpp>
#include <unordered_map>
#include <map>
#include <limits>
#include <cstddef>
namespace DB
{
/** Identifies WHERE expressions that can be placed in PREWHERE by calculating respective
* sizes of columns used in particular expression and identifying "good" conditions of
* form "column_name = constant", where "constant" is outside some `threshold` specified in advance.
*
* If there are "good" conditions present in WHERE, the one with minimal summary column size is
* transferred to PREWHERE.
* Otherwise any condition with minimal summary column size can be transferred to PREWHERE, if only
* its relative size (summary column size divided by query column size) is less than `max_columns_relative_size`.
*/
class MergeTreeWhereOptimizer
{
static constexpr auto threshold = 10;
static constexpr auto max_columns_relative_size = 0.25f;
static constexpr auto and_function_name = "and";
static constexpr auto equals_function_name = "equals";
public:
MergeTreeWhereOptimizer(const MergeTreeWhereOptimizer&) = delete;
MergeTreeWhereOptimizer& operator=(const MergeTreeWhereOptimizer&) = delete;
MergeTreeWhereOptimizer(
ASTSelectQuery & select, const MergeTreeData & data,
const Names & column_names, Logger * log)
: primary_key_columns{toUnorderedSet(data.getPrimaryExpression()->getRequiredColumnsWithTypes())},
table_columns{toUnorderedSet(data.getColumnsList())}, log{log}
{
calculateColumnSizes(data, column_names);
optimize(select);
}
private:
void optimize(ASTSelectQuery & select) const
{
if (!select.where_expression || select.prewhere_expression)
return;
const auto function = typeid_cast<ASTFunction *>(select.where_expression.get());
if (function && function->name == and_function_name)
optimizeConjunction(select, function);
else
optimizeArbitrary(select);
}
void calculateColumnSizes(const MergeTreeData & data, const Names & column_names)
{
for (const auto & column_name : column_names)
{
const auto column_size = data.getColumnSize(column_name);
column_sizes[column_name] = column_size;
total_column_size += column_size;
}
}
void optimizeConjunction(ASTSelectQuery & select, ASTFunction * const fun) const
{
/// used as max possible size and indicator that appropriate condition has not been found
const auto no_such_condition = std::numeric_limits<std::size_t>::max();
/// { first: condition index, second: summary column size }
std::pair<std::size_t, std::size_t> lightest_good_condition{no_such_condition, no_such_condition};
std::pair<std::size_t, std::size_t> lightest_viable_condition{no_such_condition, no_such_condition};
auto & conditions = fun->arguments->children;
/// remove condition by swapping it with the last one and calling ::pop_back()
const auto remove_condition_at_index = [&conditions] (const std::size_t idx) {
if (idx < conditions.size() - 1)
std::swap(conditions[idx], conditions.back());
conditions.pop_back();
};
/// linearize conjunction and divide conditions into "good" and not-"good" ones
for (std::size_t idx = 0; idx < conditions.size();)
{
const auto condition = conditions[idx].get();
IdentifierNameSet identifiers{};
collectIdentifiersNoSubqueries(condition, identifiers);
/// do not take into consideration the conditions consisting only of primary key columns
if (hasNonPrimaryKeyColumns(identifiers) && isSubsetOfTableColumns(identifiers))
{
/// linearize sub-conjunctions
if (const auto function = typeid_cast<ASTFunction *>(condition))
{
if (function->name == and_function_name)
{
for (auto & child : function->arguments->children)
conditions.emplace_back(std::move(child));
/// remove the condition corresponding to conjunction
remove_condition_at_index(idx);
/// continue iterating without increment to ensure the just added conditions are processed
continue;
}
}
/// calculate size of columns involved in condition
const auto cond_columns_size = getIdentifiersColumnSize(identifiers);
/// place condition either in good or viable conditions set
auto & good_or_viable_condition = isConditionGood(condition) ? lightest_good_condition : lightest_viable_condition;
if (good_or_viable_condition.second > cond_columns_size)
{
good_or_viable_condition.first = idx;
good_or_viable_condition.second = cond_columns_size;
}
}
++idx;
}
const auto move_condition_to_prewhere = [&] (const std::size_t idx) {
select.prewhere_expression = conditions[idx];
select.children.push_back(select.prewhere_expression);
LOG_DEBUG(log, "MergeTreeWhereOptimizer: condition `" << select.prewhere_expression << "` moved to PREWHERE");
/** Replace conjunction with the only remaining argument if only two conditions were present,
* remove selected condition from conjunction otherwise. */
if (conditions.size() == 2)
{
/// find old where_expression in children of select
const auto it = std::find(std::begin(select.children), std::end(select.children), select.where_expression);
/// replace where_expression with the remaining argument
select.where_expression = std::move(conditions[idx == 0 ? 1 : 0]);
/// overwrite child entry with the new where_expression
*it = select.where_expression;
}
else
remove_condition_at_index(idx);
};
/// if there is a "good" condition - move it to PREWHERE
if (lightest_good_condition.first != no_such_condition)
{
move_condition_to_prewhere(lightest_good_condition.first);
}
else if (lightest_viable_condition.first != no_such_condition)
{
/// check that the relative column size is less than max
if (total_column_size != 0)
{
/// calculate relative size of condition's columns
const auto cond_columns_size = lightest_viable_condition.second;
const auto columns_relative_size = static_cast<float>(cond_columns_size) / total_column_size;
/// do nothing if it exceeds max relative size
if (columns_relative_size > max_columns_relative_size)
return;
}
move_condition_to_prewhere(lightest_viable_condition.first);
}
}
void optimizeArbitrary(ASTSelectQuery & select) const
{
auto & condition = select.where_expression;
IdentifierNameSet identifiers{};
collectIdentifiersNoSubqueries(condition, identifiers);
if (!hasNonPrimaryKeyColumns(identifiers) || !isSubsetOfTableColumns(identifiers))
return;
/// if condition is not "good" - check that it can be moved
if (!isConditionGood(condition.get()) && total_column_size != 0)
{
const auto cond_columns_size = getIdentifiersColumnSize(identifiers);
const auto columns_relative_size = static_cast<float>(cond_columns_size) / total_column_size;
if (columns_relative_size > max_columns_relative_size)
return;
}
/// add the condition to PREWHERE, remove it from WHERE
std::swap(select.prewhere_expression, condition);
LOG_DEBUG(log, "MergeTreeWhereOptimizer: condition `" << select.prewhere_expression << "` moved to PREWHERE");
}
std::size_t getIdentifiersColumnSize(const IdentifierNameSet & identifiers) const
{
std::size_t size{};
for (const auto & identifier : identifiers)
if (column_sizes.count(identifier))
size += column_sizes.find(identifier)->second;
return size;
}
bool isConditionGood(const IAST * condition) const
{
const auto function = typeid_cast<const ASTFunction *>(condition);
if (!function)
return false;
/** we are only considering conditions of form `equals(one, another)` or `one = another`,
* especially if either `one` or `another` is ASTIdentifier */
if (function->name != equals_function_name)
return false;
auto left_arg = function->arguments->children.front().get();
auto right_arg = function->arguments->children.back().get();
/// try to ensure left_arg points to ASTIdentifier
if (!typeid_cast<const ASTIdentifier *>(left_arg) && typeid_cast<const ASTIdentifier *>(right_arg))
std::swap(left_arg, right_arg);
if (const auto identifier = typeid_cast<const ASTIdentifier *>(left_arg))
{
/// condition may be "good" if only right_arg is a constant and its value is outside the threshold
if (const auto literal = typeid_cast<const ASTLiteral *>(right_arg))
{
const auto & field = literal->value;
const auto type = field.getType();
/// check the value with respect to threshold
if (type == Field::Types::UInt64)
{
const auto value = field.get<UInt64>();
return value > threshold;
}
else if (type == Field::Types::Int64)
{
const auto value = field.get<Int64>();
return value < -threshold || threshold < value;
}
else if (type == Field::Types::Float64)
{
const auto value = field.get<Float64>();
return value < threshold || threshold < value;
}
}
}
return false;
}
static void collectIdentifiersNoSubqueries(const IAST * const ast, IdentifierNameSet & set)
{
if (const auto identifier = typeid_cast<const ASTIdentifier *>(ast))
return (void) set.insert(identifier->name);
if (typeid_cast<const ASTSubquery *>(ast))
return;
for (const auto & child : ast->children)
collectIdentifiersNoSubqueries(child.get(), set);
}
using string_set_t = std::unordered_set<std::string>;
static string_set_t toUnorderedSet(const NamesAndTypesList & columns)
{
string_set_t result{};
for (const auto column : columns)
result.insert(column.name);
return result;
}
bool hasNonPrimaryKeyColumns(const IdentifierNameSet & identifiers) const {
for (const auto & identifier : identifiers)
if (primary_key_columns.count(identifier) == 0)
return true;
return false;
}
bool isSubsetOfTableColumns(const IdentifierNameSet & identifiers) const {
for (const auto & identifier : identifiers)
if (table_columns.count(identifier) == 0)
return false;
return true;
}
string_set_t primary_key_columns{};
string_set_t table_columns{};
Logger * log;
std::unordered_map<std::string, std::size_t> column_sizes{};
std::size_t total_column_size{};
};
}