From 48c348f86ce09c668af7cf271757c3f156f28344 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 18 Nov 2010 11:53:49 -0500
Subject: [PATCH] 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 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.
---
src/backend/executor/nodeLimit.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index 5ed8664f66f..c939752f37a 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -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));
}
}
--
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