Function optimizePlanForExists

src/Planner/Utils.cpp:736–777

}

bool optimizePlanForExists(QueryPlan & query_plan)
{
    auto * node = query_plan.getRootNode();
    while (true)
    {
        if (typeid_cast<ExpressionStep *>(node->step.get()))
        {
            node = node->children[0];
            continue;
        }
        if (auto * aggregation = typeid_cast<AggregatingStep *>(node->step.get()))
        {
            const auto & params = aggregation->getParams();
            if (params.keys_size == 0 && !params.empty_result_for_aggregation_by_empty_set)
            {
                /// Subquery will always produce at least one row
                return true;
            }
            node = node->children[0];
            continue;
        }
        if (auto * limit_step = typeid_cast<LimitStep *>(node->step.get()))
        {
            if (limit_step->getOffset() == 0 && limit_step->getLimit() > 0)
            {
                /// TODO: Support LimitStep in decorrelation process.
                /// For now, we just remove it, because it only increases the number of rows in the result.
                /// It doesn't affect the result of correlated subquery.
                node = node->children[0];
                continue;
            }
            break;
        }
        break;
    }

    if (node != query_plan.getRootNode())
    {
        query_plan = query_plan.extractSubplan(node);
    }
    return false;
}

QueryPlanStepPtr projectOnlyUsedColumns(
    const SharedHeader & stream_header,