ClickHouse

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#include <Processors/QueryPlan/Optimizations/Optimizations.h>
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#include <Processors/QueryPlan/UnionStep.h>
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#include <Processors/QueryPlan/ExpressionStep.h>
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#include <Interpreters/ActionsDAG.h>
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namespace DB::QueryPlanOptimizations
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{
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size_t tryLiftUpUnion(QueryPlan::Node * parent_node, QueryPlan::Nodes & nodes)
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{
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    if (parent_node->children.empty())
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        return 0;
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    QueryPlan::Node * child_node = parent_node->children.front();
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    auto & parent = parent_node->step;
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    auto & child = child_node->step;
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    auto * union_step = typeid_cast<UnionStep *>(child.get());
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    if (!union_step)
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        return 0;
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    if (auto * expression = typeid_cast<ExpressionStep *>(parent.get()))
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    {
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        /// Union does not change header.
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        /// We can push down expression and update header.
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        auto union_input_streams = child->getInputStreams();
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        for (auto & input_stream : union_input_streams)
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            input_stream.header = expression->getOutputStream().header;
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        ///                    - Something
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        /// Expression - Union - Something
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        ///                    - Something
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        child = std::make_unique<UnionStep>(union_input_streams, union_step->getMaxThreads());
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        std::swap(parent, child);
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        std::swap(parent_node->children, child_node->children);
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        std::swap(parent_node->children.front(), child_node->children.front());
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        ///       - Expression - Something
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        /// Union - Something
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        ///       - Something
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        for (size_t i = 1; i < parent_node->children.size(); ++i)
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        {
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            auto & expr_node = nodes.emplace_back();
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            expr_node.children.push_back(parent_node->children[i]);
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            parent_node->children[i] = &expr_node;
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            expr_node.step = std::make_unique<ExpressionStep>(
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                expr_node.children.front()->step->getOutputStream(),
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                expression->getExpression()->clone());
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        }
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        ///       - Expression - Something
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        /// Union - Expression - Something
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        ///       - Expression - Something
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        return 3;
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    }
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    return 0;
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}
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}
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