ClickHouse/src/Storages/MergeTree/MergeTreeWhereOptimizer.h

#pragma once

#include <Core/Block.h>
#include <Interpreters/Context_fwd.h>
#include <Storages/SelectQueryInfo.h>
#include <Storages/MergeTree/RPNBuilder.h>

#include <boost/noncopyable.hpp>

#include <memory>
#include <set>
#include <unordered_map>


namespace Poco { class Logger; }

namespace DB
{

class ASTSelectQuery;
class ASTFunction;
class MergeTreeData;
struct StorageInMemoryMetadata;
using StorageMetadataPtr = std::shared_ptr<const StorageInMemoryMetadata>;

/** 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 column sizes are unknown (in compact parts), the number of columns, participating in condition is used instead.
 */
class MergeTreeWhereOptimizer : private boost::noncopyable
{
public:
    MergeTreeWhereOptimizer(
        std::unordered_map<std::string, UInt64> column_sizes_,
        const StorageMetadataPtr & metadata_snapshot,
        const Names & queried_columns_,
        const std::optional<NameSet> & supported_columns_,
        Poco::Logger * log_);

    void optimize(SelectQueryInfo & select_query_info, const ContextPtr & context) const;

    struct FilterActionsOptimizeResult
    {
        ActionsDAGPtr filter_actions;
        ActionsDAGPtr prewhere_filter_actions;
    };

    std::optional<FilterActionsOptimizeResult> optimize(const ActionsDAGPtr & filter_dag,
        const std::string & filter_column_name,
        const ContextPtr & context,
        bool is_final);

private:
    struct Condition
    {
        explicit Condition(RPNBuilderTreeNode node_)
            : node(std::move(node_))
        {}

        RPNBuilderTreeNode node;

        UInt64 columns_size = 0;
        NameSet table_columns;

        /// Can condition be moved to prewhere?
        bool viable = false;

        /// Does the condition presumably have good selectivity?
        bool good = false;

        auto tuple() const
        {
            return std::make_tuple(!viable, !good, columns_size, table_columns.size());
        }

        /// Is condition a better candidate for moving to PREWHERE?
        bool operator< (const Condition & rhs) const
        {
            return tuple() < rhs.tuple();
        }
    };

    using Conditions = std::list<Condition>;

    struct WhereOptimizerContext
    {
        ContextPtr context;
        NameSet array_joined_names;
        bool move_all_conditions_to_prewhere = false;
        bool is_final = false;
    };

    struct OptimizeResult
    {
        Conditions where_conditions;
        Conditions prewhere_conditions;
    };

    std::optional<OptimizeResult> optimizeImpl(const RPNBuilderTreeNode & node, const WhereOptimizerContext & where_optimizer_context) const;

    void analyzeImpl(Conditions & res, const RPNBuilderTreeNode & node, const WhereOptimizerContext & where_optimizer_context) const;

    /// Transform conjunctions chain in WHERE expression to Conditions list.
    Conditions analyze(const RPNBuilderTreeNode & node, const WhereOptimizerContext & where_optimizer_context) const;

    /// Reconstruct AST from conditions
    static ASTPtr reconstructAST(const Conditions & conditions);

    /// Reconstruct DAG from conditions
    static ActionsDAGPtr reconstructDAG(const Conditions & conditions, const ContextPtr & context);

    void optimizeArbitrary(ASTSelectQuery & select) const;

    UInt64 getColumnsSize(const NameSet & columns) const;

    bool columnsSupportPrewhere(const NameSet & columns) const;

    bool isExpressionOverSortingKey(const RPNBuilderTreeNode & node) const;

    bool isSortingKey(const String & column_name) const;

    bool isConstant(const ASTPtr & expr) const;

    bool isSubsetOfTableColumns(const NameSet & columns) const;

    /** ARRAY JOIN'ed columns as well as arrayJoin() result cannot be used in PREWHERE, therefore expressions
      *    containing said columns should not be moved to PREWHERE at all.
      *    We assume all AS aliases have been expanded prior to using this class
      *
      * Also, disallow moving expressions with GLOBAL [NOT] IN.
      */
    bool cannotBeMoved(const RPNBuilderTreeNode & node, const WhereOptimizerContext & where_optimizer_context) const;

    static NameSet determineArrayJoinedNames(const ASTSelectQuery & select);

    const NameSet table_columns;
    const Names queried_columns;
    const std::optional<NameSet> supported_columns;
    const NameSet sorting_key_names;
    Poco::Logger * log;
    std::unordered_map<std::string, UInt64> column_sizes;
    UInt64 total_size_of_queried_columns = 0;
};


}