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#include <Processors/QueryPlan/Optimizations/Optimizations.h>
#include <Processors/QueryPlan/ITransformingStep.h>
#include <Processors/QueryPlan/LimitStep.h>
#include <Processors/QueryPlan/TotalsHavingStep.h>
#include <Processors/QueryPlan/SortingStep.h>
#include <Processors/QueryPlan/WindowStep.h>
#include <Common/typeid_cast.h>
namespace DB::QueryPlanOptimizations
{
/// If plan looks like Limit -> Sorting, update limit for Sorting
static bool tryUpdateLimitForSortingSteps(QueryPlan::Node * node, size_t limit)
{
if (limit == 0)
return false;
QueryPlanStepPtr & step = node->step;
QueryPlan::Node * child = nullptr;
bool updated = false;
if (auto * sorting = typeid_cast<SortingStep *>(step.get()))
{
/// TODO: remove LimitStep here.
sorting->updateLimit(limit);
updated = true;
child = node->children.front();
}
/// In case we have several sorting steps.
/// Try update limit for them also if possible.
if (child)
tryUpdateLimitForSortingSteps(child, limit);
return updated;
}
size_t tryPushDownLimit(QueryPlan::Node * parent_node, QueryPlan::Nodes &)
{
if (parent_node->children.size() != 1)
return 0;
QueryPlan::Node * child_node = parent_node->children.front();
auto & parent = parent_node->step;
auto & child = child_node->step;
auto * limit = typeid_cast<LimitStep *>(parent.get());
if (!limit)
return 0;
/// Skip LIMIT WITH TIES by now.
if (limit->withTies())
return 0;
const auto * transforming = dynamic_cast<const ITransformingStep *>(child.get());
/// Skip everything which is not transform.
if (!transforming)
return 0;
/// Special cases for sorting steps.
if (tryUpdateLimitForSortingSteps(child_node, limit->getLimitForSorting()))
return 0;
/// Special case for TotalsHaving. Totals may be incorrect if we push down limit.
if (typeid_cast<const TotalsHavingStep *>(child.get()))
return 0;
/// Disable for WindowStep.
/// TODO: we can push down limit in some cases if increase the limit value.
if (typeid_cast<const WindowStep *>(child.get()))
return 0;
/// Now we should decide if pushing down limit possible for this step.
const auto & transform_traits = transforming->getTransformTraits();
const auto & data_stream_traits = transforming->getDataStreamTraits();
/// Cannot push down if child changes the number of rows.
if (!transform_traits.preserves_number_of_rows)
return 0;
/// Cannot push down if data was sorted exactly by child stream.
if (!child->getOutputStream().sort_description.empty() && !data_stream_traits.preserves_sorting)
return 0;
/// Now we push down limit only if it doesn't change any stream properties.
/// TODO: some of them may be changed and, probably, not important for following streams. We may add such info.
if (!limit->getOutputStream().hasEqualPropertiesWith(transforming->getOutputStream()))
return 0;
/// Input stream for Limit have changed.
limit->updateInputStream(transforming->getInputStreams().front());
parent.swap(child);
return 2;
}
}
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