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.

diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index 5ed8664f66f669c3d38d214203f2684d46d795fb..c939752f37a2ce2fc0b6febdb13bbf60f24cc273 100644 (file)
--- 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));
    }
 }