parallel index(-only) scan breaks when run without parallelism

Amit, Rafia,

nodeIndexscan.c, unlike nodeSeqscan.c, thinks that a parallel-aware
scan will always be executed in parallel mode. But that's not true:
an Execute message with a non-zero row count could cause us to abandon
planned parallelism and execute the plan serially. I believe this
would cause a core dump. We definitely core dump with the following
small patch, which causes parallelism to always be abandoned:

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index f5cd65d..fc4de48 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -1596,8 +1596,7 @@ ExecutePlan(EState *estate,
* when writing into a relation, because no database changes are allowed
* in parallel mode.
*/
- if (numberTuples || dest->mydest == DestIntoRel)
- use_parallel_mode = false;
+ use_parallel_mode = false;

if (use_parallel_mode)
EnterParallelMode();

I believe this defect was introduced by
5262f7a4fc44f651241d2ff1fa688dd664a34874 and that nodeIndexonlyscan.c
has the same defect as of 0414b26bac09379a4cbf1fbd847d1cee2293c5e4.

Please fix.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company