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Commit48c348f

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Dept of second thoughts: don't try to push LIMIT below a SRF.
If we have Limit->Result->Sort, the Result might be projecting a tlistthat contains a set-returning function. If so, it's possible for theSRF to sometimes return zero rows, which means we could need to fetchmore than N rows from the Sort in order to satisfy LIMIT N.So top-N sorting cannot be used in this scenario.
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‎src/backend/executor/nodeLimit.c‎

Lines changed: 14 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -23,6 +23,7 @@
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#include"executor/executor.h"
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#include"executor/nodeLimit.h"
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#include"nodes/nodeFuncs.h"
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staticvoidrecompute_limits(LimitState*node);
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staticvoidpass_down_bound(LimitState*node,PlanState*child_node);
@@ -344,7 +345,19 @@ pass_down_bound(LimitState *node, PlanState *child_node)
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}
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elseif (IsA(child_node,ResultState))
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{
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if (outerPlanState(child_node))
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/*
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* An extra consideration here is that if the Result is projecting
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* a targetlist that contains any SRFs, we can't assume that every
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* input tuple generates an output tuple, so a Sort underneath
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* might need to return more than N tuples to satisfy LIMIT N.
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* So we cannot use bounded sort.
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*
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* If Result supported qual checking, we'd have to punt on seeing
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* a qual, too. Note that having a resconstantqual is not a
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* showstopper: if that fails we're not getting any rows at all.
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*/
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if (outerPlanState(child_node)&&
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!expression_returns_set((Node*)child_node->plan->targetlist))
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pass_down_bound(node,outerPlanState(child_node));
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}
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}

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