Stop the executor if no more tuples can be sent from worker to leader.

If a Gather node has read as many tuples as it needs (for example, due
to Limit) it may detach the queue connecting it to the worker before
reading all of the worker's tuples.  Rather than let the worker
continue to generate and send all of the results, have it stop after
sending the next tuple.

More could be done here to stop the worker even quicker, but this is
about as well as we can hope to do for 9.6.

This is in response to a problem report from Andreas Seltenreich.
Commit 44339b892a04e94bbb472235882dc6f7023bdc65 should be actually be
sufficient to fix that example even without this change, but it seems
better to do this, too, since we might otherwise waste quite a large
amount of effort in one or more workers.

Discussion: CAA4eK1KOKGqmz9bGu+Z42qhRwMbm4R5rfnqsLCNqFs9j14jzEA@mail.gmail.com

Amit Kapila

3 files changed, 11 insertions, 5 deletions

diff --git a/src/backend/commands/copy.c b/src/backend/commands/copy.c
index 3201476c9e8..28dcd340017 100644
--- a/src/backend/commands/copy.c
+++ b/src/backend/commands/copy.c
@@ -4454,7 +4454,7 @@ copy_dest_startup(DestReceiver *self, int operation, TupleDesc typeinfo)
 /*
  * copy_dest_receive --- receive one tuple
  */
-static void
+static bool
 copy_dest_receive(TupleTableSlot *slot, DestReceiver *self)
 {
 	DR_copy    *myState = (DR_copy *) self;
@@ -4466,6 +4466,8 @@ copy_dest_receive(TupleTableSlot *slot, DestReceiver *self)
 	/* And send the data */
 	CopyOneRowTo(cstate, InvalidOid, slot->tts_values, slot->tts_isnull);
 	myState->processed++;
+
+	return true;
 }
 
 /*
diff --git a/src/backend/commands/createas.c b/src/backend/commands/createas.c
index cb7a145ee5d..5a853c48a8b 100644
--- a/src/backend/commands/createas.c
+++ b/src/backend/commands/createas.c
@@ -62,7 +62,7 @@ typedef struct
 static ObjectAddress CreateAsReladdr = {InvalidOid, InvalidOid, 0};
 
 static void intorel_startup(DestReceiver *self, int operation, TupleDesc typeinfo);
-static void intorel_receive(TupleTableSlot *slot, DestReceiver *self);
+static bool intorel_receive(TupleTableSlot *slot, DestReceiver *self);
 static void intorel_shutdown(DestReceiver *self);
 static void intorel_destroy(DestReceiver *self);
 
@@ -482,7 +482,7 @@ intorel_startup(DestReceiver *self, int operation, TupleDesc typeinfo)
 /*
  * intorel_receive --- receive one tuple
  */
-static void
+static bool
 intorel_receive(TupleTableSlot *slot, DestReceiver *self)
 {
 	DR_intorel *myState = (DR_intorel *) self;
@@ -507,6 +507,8 @@ intorel_receive(TupleTableSlot *slot, DestReceiver *self)
 				myState->bistate);
 
 	/* We know this is a newly created relation, so there are no indexes */
+
+	return true;
 }
 
 /*
diff --git a/src/backend/commands/matview.c b/src/backend/commands/matview.c
index f00aab39e7b..62e61a26749 100644
--- a/src/backend/commands/matview.c
+++ b/src/backend/commands/matview.c
@@ -56,7 +56,7 @@ typedef struct
 static int	matview_maintenance_depth = 0;
 
 static void transientrel_startup(DestReceiver *self, int operation, TupleDesc typeinfo);
-static void transientrel_receive(TupleTableSlot *slot, DestReceiver *self);
+static bool transientrel_receive(TupleTableSlot *slot, DestReceiver *self);
 static void transientrel_shutdown(DestReceiver *self);
 static void transientrel_destroy(DestReceiver *self);
 static void refresh_matview_datafill(DestReceiver *dest, Query *query,
@@ -467,7 +467,7 @@ transientrel_startup(DestReceiver *self, int operation, TupleDesc typeinfo)
 /*
  * transientrel_receive --- receive one tuple
  */
-static void
+static bool
 transientrel_receive(TupleTableSlot *slot, DestReceiver *self)
 {
 	DR_transientrel *myState = (DR_transientrel *) self;
@@ -486,6 +486,8 @@ transientrel_receive(TupleTableSlot *slot, DestReceiver *self)
 				myState->bistate);
 
 	/* We know this is a newly created relation, so there are no indexes */
+
+	return true;
 }
 
 /*