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

1 files changed, 8 insertions, 4 deletions

diff --git a/src/backend/executor/tstoreReceiver.c b/src/backend/executor/tstoreReceiver.c
index 516440ad32e..8f1e1b3f50c 100644
--- a/src/backend/executor/tstoreReceiver.c
+++ b/src/backend/executor/tstoreReceiver.c
@@ -37,8 +37,8 @@ typedef struct
 } TStoreState;
 
 
-static void tstoreReceiveSlot_notoast(TupleTableSlot *slot, DestReceiver *self);
-static void tstoreReceiveSlot_detoast(TupleTableSlot *slot, DestReceiver *self);
+static bool tstoreReceiveSlot_notoast(TupleTableSlot *slot, DestReceiver *self);
+static bool tstoreReceiveSlot_detoast(TupleTableSlot *slot, DestReceiver *self);
 
 
 /*
@@ -90,19 +90,21 @@ tstoreStartupReceiver(DestReceiver *self, int operation, TupleDesc typeinfo)
  * Receive a tuple from the executor and store it in the tuplestore.
  * This is for the easy case where we don't have to detoast.
  */
-static void
+static bool
 tstoreReceiveSlot_notoast(TupleTableSlot *slot, DestReceiver *self)
 {
 	TStoreState *myState = (TStoreState *) self;
 
 	tuplestore_puttupleslot(myState->tstore, slot);
+
+	return true;
 }
 
 /*
  * Receive a tuple from the executor and store it in the tuplestore.
  * This is for the case where we have to detoast any toasted values.
  */
-static void
+static bool
 tstoreReceiveSlot_detoast(TupleTableSlot *slot, DestReceiver *self)
 {
 	TStoreState *myState = (TStoreState *) self;
@@ -152,6 +154,8 @@ tstoreReceiveSlot_detoast(TupleTableSlot *slot, DestReceiver *self)
 	/* And release any temporary detoasted values */
 	for (i = 0; i < nfree; i++)
 		pfree(DatumGetPointer(myState->tofree[i]));
+
+	return true;
 }
 
 /*