ClickHouse

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#include <Processors/QueryPlan/Optimizations/Optimizations.h>
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#include <Processors/QueryPlan/FilterStep.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 tryMergeExpressions(QueryPlan::Node * parent_node, QueryPlan::Nodes &)
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{
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    if (parent_node->children.size() != 1)
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        return false;
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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 * parent_expr = typeid_cast<ExpressionStep *>(parent.get());
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    auto * parent_filter = typeid_cast<FilterStep *>(parent.get());
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    auto * child_expr = typeid_cast<ExpressionStep *>(child.get());
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    if (parent_expr && child_expr)
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    {
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        const auto & child_actions = child_expr->getExpression();
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        const auto & parent_actions = parent_expr->getExpression();
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        /// We cannot combine actions with arrayJoin and stateful function because we not always can reorder them.
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        /// Example: select rowNumberInBlock() from (select arrayJoin([1, 2]))
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        /// Such a query will return two zeroes if we combine actions together.
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        if (child_actions->hasArrayJoin() && parent_actions->hasStatefulFunctions())
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            return 0;
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        auto merged = ActionsDAG::merge(std::move(*child_actions), std::move(*parent_actions));
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        auto expr = std::make_unique<ExpressionStep>(child_expr->getInputStreams().front(), merged);
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        expr->setStepDescription("(" + parent_expr->getStepDescription() + " + " + child_expr->getStepDescription() + ")");
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        parent_node->step = std::move(expr);
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        parent_node->children.swap(child_node->children);
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        return 1;
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    }
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    else if (parent_filter && child_expr)
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    {
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        const auto & child_actions = child_expr->getExpression();
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        const auto & parent_actions = parent_filter->getExpression();
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        if (child_actions->hasArrayJoin() && parent_actions->hasStatefulFunctions())
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            return 0;
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        auto merged = ActionsDAG::merge(std::move(*child_actions), std::move(*parent_actions));
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        auto filter = std::make_unique<FilterStep>(child_expr->getInputStreams().front(),
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                                                   merged,
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                                                   parent_filter->getFilterColumnName(),
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                                                   parent_filter->removesFilterColumn());
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        filter->setStepDescription("(" + parent_filter->getStepDescription() + " + " + child_expr->getStepDescription() + ")");
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        parent_node->step = std::move(filter);
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        parent_node->children.swap(child_node->children);
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        return 1;
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    }
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    return 0;
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}
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}
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