Method getSampleBlockImpl

src/Interpreters/InterpreterSelectQuery.cpp:1246–1379

}

Block InterpreterSelectQuery::getSampleBlockImpl()
{
    auto & select_query = getSelectQuery();

    query_info.query = query_ptr;

    /// NOTE: this is required for getQueryProcessingStage(), so should be initialized before ExpressionAnalysisResult.
    query_info.has_window = query_analyzer->hasWindow();
    /// NOTE: this is required only for IStorage::read(), and to be precise MergeTreeData::read(), in case of projections.
    query_info.has_order_by = select_query.orderBy() != nullptr;
    query_info.need_aggregate = query_analyzer->hasAggregation();

    if (storage && !options.only_analyze)
    {
        query_info.prepared_sets = query_analyzer->getPreparedSets();
        from_stage = storage->getQueryProcessingStage(context, options.to_stage, storage_snapshot, query_info);
    }

    /// Do I need to perform the first part of the pipeline?
    /// Running on remote servers during distributed processing or if query is not distributed.
    ///
    /// Also note that with distributed_group_by_no_merge=1 or when there is
    /// only one remote server, it is equal to local query in terms of query
    /// stages (or when due to optimize_distributed_group_by_sharding_key the query was processed up to Complete stage).
    bool first_stage = from_stage < QueryProcessingStage::WithMergeableState
        && options.to_stage >= QueryProcessingStage::WithMergeableState;
    /// Do I need to execute the second part of the pipeline?
    /// Running on the initiating server during distributed processing or if query is not distributed.
    ///
    /// Also note that with distributed_group_by_no_merge=2 (i.e. when optimize_distributed_group_by_sharding_key takes place)
    /// the query on the remote server will be processed up to WithMergeableStateAfterAggregationAndLimit,
    /// So it will do partial second stage (second_stage=true), and initiator will do the final part.
    bool second_stage = from_stage <= QueryProcessingStage::WithMergeableState
        && options.to_stage > QueryProcessingStage::WithMergeableState;

    analysis_result = ExpressionAnalysisResult(
        *query_analyzer, metadata_snapshot, first_stage, second_stage, options.only_analyze, row_policy_info, additional_filter_info, *source_header);

    if (options.to_stage == QueryProcessingStage::Enum::FetchColumns)
    {
        auto header = *source_header;

        if (analysis_result.prewhere_info)
        {
            header = analysis_result.prewhere_info->prewhere_actions.updateHeader(header);
            if (analysis_result.prewhere_info->remove_prewhere_column)
                header.erase(analysis_result.prewhere_info->prewhere_column_name);
        }
        return header;
    }

    if (options.to_stage == QueryProcessingStage::Enum::WithMergeableState)
    {
        if (!analysis_result.need_aggregate)
        {
            // What's the difference with selected_columns?
            // Here we calculate the header we want from remote server after it
            // executes query up to WithMergeableState. When there is an ORDER BY,