Teach bitmap heap scan to cope with absence of a DSA.

If we have a plan that uses parallelism but are unable to execute it
using parallelism, for example due to a lack of available DSM
segments, then the EState's es_query_dsa will be NULL.  Parallel
bitmap heap scan needs to fall back to a non-parallel scan in such
cases.

Patch by me, reviewed by Dilip Kumar

Discussion: http://postgr.es/m/CAEepm=0kADK5inNf_KuemjX=HQ=PuTP0DykM--fO5jS5ePVFEA@mail.gmail.com

src/backend/executor/nodeBitmapHeapscan.c

@@ -1051,6 +1051,11 @@ ExecBitmapHeapInitializeDSM(BitmapHeapScanState *node,
 {
 	ParallelBitmapHeapState *pstate;
 	EState	   *estate = node->ss.ps.state;
+	dsa_area   *dsa = node->ss.ps.state->es_query_dsa;
+
+	/* If there's no DSA, there are no workers; initialize nothing. */
+	if (dsa == NULL)
+		return;
 
 	pstate = shm_toc_allocate(pcxt->toc, node->pscan_len);
 
@@ -1083,6 +1088,10 @@ ExecBitmapHeapReInitializeDSM(BitmapHeapScanState *node,
 	ParallelBitmapHeapState *pstate = node->pstate;
 	dsa_area   *dsa = node->ss.ps.state->es_query_dsa;
 
+	/* If there's no DSA, there are no workers; do nothing. */
+	if (dsa == NULL)
+		return;
+
 	pstate->state = BM_INITIAL;
 
 	if (DsaPointerIsValid(pstate->tbmiterator))
@@ -1108,6 +1117,8 @@ ExecBitmapHeapInitializeWorker(BitmapHeapScanState *node,
 	ParallelBitmapHeapState *pstate;
 	Snapshot	snapshot;
 
+	Assert(node->ss.ps.state->es_query_dsa != NULL);
+
 	pstate = shm_toc_lookup(pwcxt->toc, node->ss.ps.plan->plan_node_id, false);
 	node->pstate = pstate;