ClickHouse/dbms/src/Interpreters/MutationsInterpreter.h

#pragma once

#include <Interpreters/ExpressionActions.h>
#include <Interpreters/InterpreterSelectQuery.h>
#include <DataStreams/IBlockInputStream.h>
#include <Storages/IStorage.h>
#include <Storages/MutationCommands.h>


namespace DB
{

class Context;

/// Create an input stream that will read data from storage and apply mutation commands (UPDATEs, DELETEs)
/// to this data.
class MutationsInterpreter
{
public:
    MutationsInterpreter(StoragePtr storage_, std::vector<MutationCommand> commands_, const Context & context_)
        : storage(std::move(storage_))
        , commands(std::move(commands_))
        , context(context_)
    {
    }

    /// Return false if the data isn't going to be changed by mutations.
    bool isStorageTouchedByMutations() const;

    /// The resulting stream will return blocks containing changed columns only.
    BlockInputStreamPtr execute();

private:
    void prepare();

private:
    StoragePtr storage;
    std::vector<MutationCommand> commands;
    const Context & context;

    /// A sequence of mutation commands is executed as a sequence of stages. Each stage consists of several
    /// DELETEs, possibly followed by an UPDATE. Commands can reuse expressions calculated by the previous
    /// commands in the same stage, but at the end of each stage intermediate columns are thrown away
    /// (they may contain wrong values because of an UPDATE).
    ///
    /// Each stage has output_columns that contain columns that are changed at the end of that stage
    /// plus columns needed for the next mutations.
    ///
    /// First stage is special: it can contain only DELETEs and is executed using InterpreterSelectQuery
    /// to take advantage of table indexes (if there are any).
    struct Stage
    {
        std::vector<MutationCommand> deletes;
        std::optional<MutationCommand> update;

        /// Contains columns that are changed by this stage,
        /// columns changed by the previous stages and also columns needed by the next stages.
        NameSet output_columns;

        /// A chain of actions needed to execute this stage.
        /// First steps calculate filter columns for DELETEs (in the same order as in `delete_filter_column_names`),
        /// then there is (possibly) an UPDATE stage, and finally a projection stage.
        ExpressionActionsChain expressions_chain;
        Names delete_filter_column_names;
    };

    std::unique_ptr<InterpreterSelectQuery> interpreter_select;
    std::vector<Stage> stages;
    bool is_prepared = false; /// Has the sequence of stages been prepared.
};

}