On Sat, Mar 25, 2017 at 7:01 PM, Peter Eisentraut <
> On 3/25/17 09:01, David Rowley wrote:
> > On 25 March 2017 at 23:09, Rushabh Lathia <rushabh.lat...@gmail.com>
> >> Also another point which I think we should fix is, when someone set
> >> max_parallel_workers = 0, we should also set the
> >> max_parallel_workers_per_gather
> >> to zero. So that way it we can avoid generating the gather path with
> >> max_parallel_worker = 0.
> > I see that it was actually quite useful that it works the way it does.
> > If it had worked the same as max_parallel_workers_per_gather, then
> > likely Tomas would never have found this bug.
> Another problem is that the GUC system doesn't really support cases
> where the validity of one setting depends on the current value of
> another setting. So each individual setting needs to be robust against
> cases of related settings being nonsensical.
About the original issue reported by Tomas, I did more debugging and
found that - problem was gather_merge_clear_slots() was not returning
the clear slot when nreader is zero (means nworkers_launched = 0).
Due to the same scan was continue even all the tuple are exhausted,
and then end up with server crash at gather_merge_getnext(). In the patch
I also added the Assert into gather_merge_getnext(), about the index
should be less then the nreaders + 1 (leader).
PFA simple patch to fix the problem.
diff --git a/src/backend/executor/nodeGatherMerge.c b/src/backend/executor/nodeGatherMerge.c
index 72f30ab..e19bace 100644
@@ -431,9 +431,14 @@ gather_merge_clear_slots(GatherMergeState *gm_state)
- for (i = 0; i < gm_state->nreaders; i++)
+ for (i = 0; i < gm_state->nreaders + 1; i++)
+ * Gather merge always allow leader to participate and we don't
+ * allocate the tuple, so no need to free the tuple for leader.
+ if (i != gm_state->nreaders)
gm_state->gm_slots[i] = ExecClearTuple(gm_state->gm_slots[i]);
@@ -474,6 +479,8 @@ gather_merge_getnext(GatherMergeState *gm_state)
i = DatumGetInt32(binaryheap_first(gm_state->gm_heap));
+ Assert(i < gm_state->nreaders + 1);
if (gather_merge_readnext(gm_state, i, false))
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