example-execprocnodefirst-patch.diff

diff --git a/src/backend/executor/execProcnode.c b/src/backend/executor/execProcnode.c
index f5f9cfbeead..536193e11d4 100644
--- a/src/backend/executor/execProcnode.c
+++ b/src/backend/executor/execProcnode.c
@@ -441,21 +441,37 @@ ExecSetExecProcNode(PlanState *node, ExecProcNodeMtd function)
 
 
 /*
- * ExecProcNode wrapper that performs some one-time checks, before calling
+ * ExecProcNode wrapper that performs some extra checks, before calling
  * the relevant node method (possibly via an instrumentation wrapper).
+ *
+ * Normally, this is just invoked once for the first call to any given node,
+ * and thereafter we arrange to call ExecProcNodeInstr or the relevant node
+ * method directly. However, it's legal to reset node->ExecProcNode back to
+ * this function at any time, and we do that whenever the query plan might
+ * need to be printed, so that we only incur the cost of checking for that
+ * case when required.
  */
 static TupleTableSlot *
 ExecProcNodeFirst(PlanState *node)
 {
 	/*
-	 * Perform stack depth check during the first execution of the node.  We
-	 * only do so the first time round because it turns out to not be cheap on
-	 * some common architectures (eg. x86).  This relies on the assumption
-	 * that ExecProcNode calls for a given plan node will always be made at
-	 * roughly the same stack depth.
+	 * Perform a stack depth check.  We don't want to do this all the time
+	 * because it turns out to not be cheap on some common architectures
+	 * (eg. x86).  This relies on the assumption that ExecProcNode calls for
+	 * a given plan node will always be made at roughly the same stack depth.
 	 */
 	check_stack_depth();
 
+	/*
+	 * If we have been asked to print the query plan, do that now. We dare
+	 * not try to do this directly from CHECK_FOR_INTERRUPTS() because we
+	 * don't really know what the executor state is at that point, but we
+	 * assume that when entering a node the state will be sufficiently
+	 * consistent that trying to print the plan makes sense.
+	 */
+	if (LogQueryPlanPending)
+		LogQueryPlan(node);
+
 	/*
 	 * If instrumentation is required, change the wrapper to one that just
 	 * does instrumentation.  Otherwise we can dispense with all wrappers and