Remove redundant snapshot copying from parallel leader to workers

The parallel query infrastructure copies the leader backend's active
snapshot to the worker processes. But BitmapHeapScan node also had
bespoken code to pass the snapshot from leader to the worker. That was
redundant, so remove it.

The removed code was analogous to the snapshot serialization in
table_parallelscan_initialize(), but that was the wrong role model. A
parallel bitmap heap scan is more like an independent non-parallel
bitmap heap scan in each parallel worker as far as the table AM is
concerned, because the coordination is done in nodeBitmapHeapscan.c,
and the table AM doesn't need to know anything about it.

This relies on the assumption that es_snapshot ==
GetActiveSnapshot(). That's not a new assumption, things would get
weird if you used the QueryDesc's snapshot for visibility checks in
the scans, but the active snapshot for evaluating quals, for
example. This could use some refactoring and cleanup, but for now,
just add some assertions.

Reviewed-by: Dilip Kumar, Robert Haas
Discussion: https://www.postgresql.org/message-id/5f3b9d59-0f43-419d-80ca-6d04c07cf61a@iki.fi

src/backend/access/table/tableam.c

@@ -120,16 +120,6 @@ table_beginscan_catalog(Relation relation, int nkeys, struct ScanKeyData *key)
 											NULL, flags);
 }
 
-void
-table_scan_update_snapshot(TableScanDesc scan, Snapshot snapshot)
-{
-	Assert(IsMVCCSnapshot(snapshot));
-
-	RegisterSnapshot(snapshot);
-	scan->rs_snapshot = snapshot;
-	scan->rs_flags |= SO_TEMP_SNAPSHOT;
-}
-
 
 /* ----------------------------------------------------------------------------
  * Parallel table scan related functions.

src/backend/executor/execMain.c

@@ -147,6 +147,9 @@ standard_ExecutorStart(QueryDesc *queryDesc, int eflags)
 	Assert(queryDesc != NULL);
 	Assert(queryDesc->estate == NULL);
 
+	/* caller must ensure the query's snapshot is active */
+	Assert(GetActiveSnapshot() == queryDesc->snapshot);
+
 	/*
 	 * If the transaction is read-only, we need to check if any writes are
 	 * planned to non-temporary tables.  EXPLAIN is considered read-only.
@@ -319,6 +322,9 @@ standard_ExecutorRun(QueryDesc *queryDesc,
 	Assert(estate != NULL);
 	Assert(!(estate->es_top_eflags & EXEC_FLAG_EXPLAIN_ONLY));
 
+	/* caller must ensure the query's snapshot is active */
+	Assert(GetActiveSnapshot() == estate->es_snapshot);
+
 	/*
 	 * Switch into per-query memory context
 	 */

src/backend/executor/execParallel.c

@@ -720,6 +720,13 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate,
 	shm_toc_estimate_chunk(&pcxt->estimator, dsa_minsize);
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
+	/*
+	 * InitializeParallelDSM() passes the active snapshot to the parallel
+	 * worker, which uses it to set es_snapshot.  Make sure we don't set
+	 * es_snapshot differently in the child.
+	 */
+	Assert(GetActiveSnapshot() == estate->es_snapshot);
+
 	/* Everyone's had a chance to ask for space, so now create the DSM. */
 	InitializeParallelDSM(pcxt);
 

src/backend/executor/nodeBitmapHeapscan.c

@@ -721,7 +721,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 	scanstate->prefetch_iterator = NULL;
 	scanstate->prefetch_pages = 0;
 	scanstate->prefetch_target = 0;
-	scanstate->pscan_len = 0;
 	scanstate->initialized = false;
 	scanstate->shared_tbmiterator = NULL;
 	scanstate->shared_prefetch_iterator = NULL;
@@ -841,13 +840,7 @@ void
 ExecBitmapHeapEstimate(BitmapHeapScanState *node,
 					   ParallelContext *pcxt)
 {
-	EState	   *estate = node->ss.ps.state;
-
-	node->pscan_len = add_size(offsetof(ParallelBitmapHeapState,
-										phs_snapshot_data),
-							   EstimateSnapshotSpace(estate->es_snapshot));
-
-	shm_toc_estimate_chunk(&pcxt->estimator, node->pscan_len);
+	shm_toc_estimate_chunk(&pcxt->estimator, sizeof(ParallelBitmapHeapState));
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 }
 
@@ -862,14 +855,13 @@ ExecBitmapHeapInitializeDSM(BitmapHeapScanState *node,
 							ParallelContext *pcxt)
 {
 	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);
+	pstate = shm_toc_allocate(pcxt->toc, sizeof(ParallelBitmapHeapState));
 
 	pstate->tbmiterator = 0;
 	pstate->prefetch_iterator = 0;
@@ -881,7 +873,6 @@ ExecBitmapHeapInitializeDSM(BitmapHeapScanState *node,
 	pstate->state = BM_INITIAL;
 
 	ConditionVariableInit(&pstate->cv);
-	SerializeSnapshot(estate->es_snapshot, pstate->phs_snapshot_data);
 
 	shm_toc_insert(pcxt->toc, node->ss.ps.plan->plan_node_id, pstate);
 	node->pstate = pstate;
@@ -927,13 +918,9 @@ ExecBitmapHeapInitializeWorker(BitmapHeapScanState *node,
 							   ParallelWorkerContext *pwcxt)
 {
 	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;
-
-	snapshot = RestoreSnapshot(pstate->phs_snapshot_data);
-	table_scan_update_snapshot(node->ss.ss_currentScanDesc, snapshot);
 }

src/include/access/tableam.h

@@ -1038,11 +1038,6 @@ table_rescan_set_params(TableScanDesc scan, struct ScanKeyData *key,
 										 allow_pagemode);
 }
 
-/*
- * Update snapshot used by the scan.
- */
-extern void table_scan_update_snapshot(TableScanDesc scan, Snapshot snapshot);
-
 /*
  * Return next tuple from `scan`, store in slot.
  */

src/include/nodes/execnodes.h

@@ -1689,7 +1689,6 @@ typedef enum
  *		prefetch_target			current target prefetch distance
  *		state					current state of the TIDBitmap
  *		cv						conditional wait variable
- *		phs_snapshot_data		snapshot data shared to workers
  * ----------------
  */
 typedef struct ParallelBitmapHeapState
@@ -1701,7 +1700,6 @@ typedef struct ParallelBitmapHeapState
 	int			prefetch_target;
 	SharedBitmapState state;
 	ConditionVariable cv;
-	char		phs_snapshot_data[FLEXIBLE_ARRAY_MEMBER];
 } ParallelBitmapHeapState;
 
 /* ----------------
@@ -1721,7 +1719,6 @@ typedef struct ParallelBitmapHeapState
  *		prefetch_pages	   # pages prefetch iterator is ahead of current
  *		prefetch_target    current target prefetch distance
  *		prefetch_maximum   maximum value for prefetch_target
- *		pscan_len		   size of the shared memory for parallel bitmap
  *		initialized		   is node is ready to iterate
  *		shared_tbmiterator	   shared iterator
  *		shared_prefetch_iterator shared iterator for prefetching
@@ -1745,7 +1742,6 @@ typedef struct BitmapHeapScanState
 	int			prefetch_pages;
 	int			prefetch_target;
 	int			prefetch_maximum;
-	Size		pscan_len;
 	bool		initialized;
 	TBMSharedIterator *shared_tbmiterator;
 	TBMSharedIterator *shared_prefetch_iterator;