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Dept of second thoughts: don't try to push LIMIT below a SRF.

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If we have Limit->Result->Sort, the Result might be projecting a tlist
that contains a set-returning function.  If so, it's possible for the
SRF to sometimes return zero rows, which means we could need to fetch
more than N rows from the Sort in order to satisfy LIMIT N.
So top-N sorting cannot be used in this scenario.
This commit is contained in:
Tom Lane 2010-11-18 11:53:49 -05:00
parent 1fc2d60d8c
commit 48c348f86c
1 changed files with 14 additions and 1 deletions
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15
src/backend/executor/nodeLimit.c
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@ -23,6 +23,7 @@
#include "executor/executor.h"
#include "executor/nodeLimit.h"
#include "nodes/nodeFuncs.h"
static void recompute_limits(LimitState *node);
static void pass_down_bound(LimitState *node, PlanState *child_node);
@ -344,7 +345,19 @@ pass_down_bound(LimitState *node, PlanState *child_node)
}
else if (IsA(child_node, ResultState))
{
if (outerPlanState(child_node))
/*
* An extra consideration here is that if the Result is projecting
* a targetlist that contains any SRFs, we can't assume that every
* input tuple generates an output tuple, so a Sort underneath
* might need to return more than N tuples to satisfy LIMIT N.
* So we cannot use bounded sort.
*
* If Result supported qual checking, we'd have to punt on seeing
* a qual, too. Note that having a resconstantqual is not a
* showstopper: if that fails we're not getting any rows at all.
*/
if (outerPlanState(child_node) &&
!expression_returns_set((Node *) child_node->plan->targetlist))
pass_down_bound(node, outerPlanState(child_node));
}
}