Remove dead NoMovementScanDirection code

Here remove some dead code from heapgettup() and heapgettup_pagemode()
which was trying to support NoMovementScanDirection scans.  This code can
never be reached as standard_ExecutorRun() never calls ExecutePlan with
NoMovementScanDirection.

Additionally, plans which were scanning an unordered index would use
NoMovementScanDirection rather than ForwardScanDirection.  There was no
real need for this, so here we adjust this so we use ForwardScanDirection
for unordered index scans.  A comment in pathnodes.h claimed that
NoMovementScanDirection was used for PathKey reasons, but if that was
true, it no longer is, per code in build_index_paths().

This does change the non-text format of the EXPLAIN output so that
unordered index scans now have a "Forward" scan direction rather than
"NoMovement".  The text format of EXPLAIN has not changed.

Author: Melanie Plageman
Reviewed-by: Tom Lane, David Rowley
Discussion: https://postgr.es/m/CAAKRu_bvkhka0CZQun28KTqhuUh5ZqY=_T8QEqZqOL02rpi2bw@mail.gmail.com

Author: david-rowley

src/backend/access/heap/heapam.c

@@ -490,9 +490,6 @@ heapgetpage(TableScanDesc sscan, BlockNumber block)
  *		tuple as indicated by "dir"; return the next tuple in scan->rs_ctup,
  *		or set scan->rs_ctup.t_data = NULL if no more tuples.
  *
- * dir == NoMovementScanDirection means "re-fetch the tuple indicated
- * by scan->rs_ctup".
- *
  * Note: the reason nkeys/key are passed separately, even though they are
  * kept in the scan descriptor, is that the caller may not want us to check
  * the scankeys.
@@ -583,7 +580,7 @@ heapgettup(HeapScanDesc scan,
 
 		linesleft = lines - lineoff + 1;
 	}
-	else if (backward)
+	else
 	{
 		/* backward parallel scan not supported */
 		Assert(scan->rs_base.rs_parallel == NULL);
@@ -653,34 +650,6 @@ heapgettup(HeapScanDesc scan,
 
 		linesleft = lineoff;
 	}
-	else
-	{
-		/*
-		 * ``no movement'' scan direction: refetch prior tuple
-		 */
-		if (!scan->rs_inited)
-		{
-			Assert(!BufferIsValid(scan->rs_cbuf));
-			tuple->t_data = NULL;
-			return;
-		}
-
-		block = ItemPointerGetBlockNumber(&(tuple->t_self));
-		if (block != scan->rs_cblock)
-			heapgetpage((TableScanDesc) scan, block);
-
-		/* Since the tuple was previously fetched, needn't lock page here */
-		page = BufferGetPage(scan->rs_cbuf);
-		TestForOldSnapshot(snapshot, scan->rs_base.rs_rd, page);
-		lineoff = ItemPointerGetOffsetNumber(&(tuple->t_self));
-		lpp = PageGetItemId(page, lineoff);
-		Assert(ItemIdIsNormal(lpp));
-
-		tuple->t_data = (HeapTupleHeader) PageGetItem(page, lpp);
-		tuple->t_len = ItemIdGetLength(lpp);
-
-		return;
-	}
 
 	/*
 	 * advance the scan until we find a qualifying tuple or run out of stuff
@@ -918,7 +887,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 
 		linesleft = lines - lineindex;
 	}
-	else if (backward)
+	else
 	{
 		/* backward parallel scan not supported */
 		Assert(scan->rs_base.rs_parallel == NULL);
@@ -978,38 +947,6 @@ heapgettup_pagemode(HeapScanDesc scan,
 
 		linesleft = lineindex + 1;
 	}
-	else
-	{
-		/*
-		 * ``no movement'' scan direction: refetch prior tuple
-		 */
-		if (!scan->rs_inited)
-		{
-			Assert(!BufferIsValid(scan->rs_cbuf));
-			tuple->t_data = NULL;
-			return;
-		}
-
-		block = ItemPointerGetBlockNumber(&(tuple->t_self));
-		if (block != scan->rs_cblock)
-			heapgetpage((TableScanDesc) scan, block);
-
-		/* Since the tuple was previously fetched, needn't lock page here */
-		page = BufferGetPage(scan->rs_cbuf);
-		TestForOldSnapshot(scan->rs_base.rs_snapshot, scan->rs_base.rs_rd, page);
-		lineoff = ItemPointerGetOffsetNumber(&(tuple->t_self));
-		lpp = PageGetItemId(page, lineoff);
-		Assert(ItemIdIsNormal(lpp));
-
-		tuple->t_data = (HeapTupleHeader) PageGetItem(page, lpp);
-		tuple->t_len = ItemIdGetLength(lpp);
-
-		/* check that rs_cindex is in sync */
-		Assert(scan->rs_cindex < scan->rs_ntuples);
-		Assert(lineoff == scan->rs_vistuples[scan->rs_cindex]);
-
-		return;
-	}
 
 	/*
 	 * advance the scan until we find a qualifying tuple or run out of stuff

src/backend/commands/explain.c

@@ -3746,9 +3746,6 @@ ExplainIndexScanDetails(Oid indexid, ScanDirection indexorderdir,
 			case BackwardScanDirection:
 				scandir = "Backward";
 				break;
-			case NoMovementScanDirection:
-				scandir = "NoMovement";
-				break;
 			case ForwardScanDirection:
 				scandir = "Forward";
 				break;

src/backend/executor/nodeIndexonlyscan.c

@@ -70,15 +70,13 @@ IndexOnlyNext(IndexOnlyScanState *node)
 	 * extract necessary information from index scan node
 	 */
 	estate = node->ss.ps.state;
-	direction = estate->es_direction;
-	/* flip direction if this is an overall backward scan */
-	if (ScanDirectionIsBackward(((IndexOnlyScan *) node->ss.ps.plan)->indexorderdir))
-	{
-		if (ScanDirectionIsForward(direction))
-			direction = BackwardScanDirection;
-		else if (ScanDirectionIsBackward(direction))
-			direction = ForwardScanDirection;
-	}
+
+	/*
+	 * Determine which direction to scan the index in based on the plan's scan
+	 * direction and the current direction of execution.
+	 */
+	direction = ScanDirectionCombine(estate->es_direction,
+									 ((IndexOnlyScan *) node->ss.ps.plan)->indexorderdir);
 	scandesc = node->ioss_ScanDesc;
 	econtext = node->ss.ps.ps_ExprContext;
 	slot = node->ss.ss_ScanTupleSlot;

src/backend/executor/nodeIndexscan.c

@@ -90,15 +90,13 @@ IndexNext(IndexScanState *node)
 	 * extract necessary information from index scan node
 	 */
 	estate = node->ss.ps.state;
-	direction = estate->es_direction;
-	/* flip direction if this is an overall backward scan */
-	if (ScanDirectionIsBackward(((IndexScan *) node->ss.ps.plan)->indexorderdir))
-	{
-		if (ScanDirectionIsForward(direction))
-			direction = BackwardScanDirection;
-		else if (ScanDirectionIsBackward(direction))
-			direction = ForwardScanDirection;
-	}
+
+	/*
+	 * Determine which direction to scan the index in based on the plan's scan
+	 * direction and the current direction of execution.
+	 */
+	direction = ScanDirectionCombine(estate->es_direction,
+									 ((IndexScan *) node->ss.ps.plan)->indexorderdir);
 	scandesc = node->iss_ScanDesc;
 	econtext = node->ss.ps.ps_ExprContext;
 	slot = node->ss.ss_ScanTupleSlot;

src/backend/optimizer/path/indxpath.c

@@ -1015,9 +1015,7 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
 								  orderbyclauses,
 								  orderbyclausecols,
 								  useful_pathkeys,
-								  index_is_ordered ?
-								  ForwardScanDirection :
-								  NoMovementScanDirection,
+								  ForwardScanDirection,
 								  index_only_scan,
 								  outer_relids,
 								  loop_count,
@@ -1037,9 +1035,7 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
 									  orderbyclauses,
 									  orderbyclausecols,
 									  useful_pathkeys,
-									  index_is_ordered ?
-									  ForwardScanDirection :
-									  NoMovementScanDirection,
+									  ForwardScanDirection,
 									  index_only_scan,
 									  outer_relids,
 									  loop_count,

src/backend/optimizer/plan/createplan.c

@@ -3017,6 +3017,9 @@ create_indexscan_plan(PlannerInfo *root,
 	/* it should be a base rel... */
 	Assert(baserelid > 0);
 	Assert(best_path->path.parent->rtekind == RTE_RELATION);
+	/* check the scan direction is valid */
+	Assert(best_path->indexscandir == ForwardScanDirection ||
+		   best_path->indexscandir == BackwardScanDirection);
 
 	/*
 	 * Extract the index qual expressions (stripped of RestrictInfos) from the

src/backend/optimizer/util/pathnode.c

@@ -982,9 +982,7 @@ create_samplescan_path(PlannerInfo *root, RelOptInfo *rel, Relids required_outer
  * 'indexorderbycols' is an integer list of index column numbers (zero based)
  *			the ordering operators can be used with.
  * 'pathkeys' describes the ordering of the path.
- * 'indexscandir' is ForwardScanDirection or BackwardScanDirection
- *			for an ordered index, or NoMovementScanDirection for
- *			an unordered index.
+ * 'indexscandir' is either ForwardScanDirection or BackwardScanDirection.
  * 'indexonly' is true if an index-only scan is wanted.
  * 'required_outer' is the set of outer relids for a parameterized path.
  * 'loop_count' is the number of repetitions of the indexscan to factor into

src/include/access/sdir.h

@@ -16,8 +16,10 @@
 
 
 /*
- * ScanDirection was an int8 for no apparent reason. I kept the original
- * values because I'm not sure if I'll break anything otherwise.  -ay 2/95
+ * Defines the direction for scanning a table or an index.  Scans are never
+ * invoked using NoMovementScanDirectionScans.  For convenience, we use the
+ * values -1 and 1 for backward and forward scans.  This allows us to perform
+ * a few mathematical tricks such as what is done in ScanDirectionCombine.
  */
 typedef enum ScanDirection
 {
@@ -26,6 +28,13 @@ typedef enum ScanDirection
 	ForwardScanDirection = 1
 } ScanDirection;
 
+/*
+ * Determine the net effect of two direction specifications.
+ * This relies on having ForwardScanDirection = +1, BackwardScanDirection = -1,
+ * and will probably not do what you want if applied to any other values.
+ */
+#define ScanDirectionCombine(a, b)  ((a) * (b))
+
 /*
  * ScanDirectionIsValid
  *		True iff scan direction is valid.

src/include/access/tableam.h

@@ -1035,6 +1035,10 @@ table_scan_getnextslot(TableScanDesc sscan, ScanDirection direction, TupleTableS
 {
 	slot->tts_tableOid = RelationGetRelid(sscan->rs_rd);
 
+	/* We don't expect actual scans using NoMovementScanDirection */
+	Assert(direction == ForwardScanDirection ||
+		   direction == BackwardScanDirection);
+
 	/*
 	 * We don't expect direct calls to table_scan_getnextslot with valid
 	 * CheckXidAlive for catalog or regular tables.  See detailed comments in
@@ -1099,6 +1103,10 @@ table_scan_getnextslot_tidrange(TableScanDesc sscan, ScanDirection direction,
 	/* Ensure table_beginscan_tidrange() was used. */
 	Assert((sscan->rs_flags & SO_TYPE_TIDRANGESCAN) != 0);
 
+	/* We don't expect actual scans using NoMovementScanDirection */
+	Assert(direction == ForwardScanDirection ||
+		   direction == BackwardScanDirection);
+
 	return sscan->rs_rd->rd_tableam->scan_getnextslot_tidrange(sscan,
 															   direction,
 															   slot);

src/include/nodes/pathnodes.h

@@ -1672,12 +1672,9 @@ typedef struct Path
  * on which index column each ORDER BY can be used with.)
  *
  * 'indexscandir' is one of:
- *		ForwardScanDirection: forward scan of an ordered index
+ *		ForwardScanDirection: forward scan of an index
  *		BackwardScanDirection: backward scan of an ordered index
- *		NoMovementScanDirection: scan of an unordered index, or don't care
- * (The executor doesn't care whether it gets ForwardScanDirection or
- * NoMovementScanDirection for an indexscan, but the planner wants to
- * distinguish ordered from unordered indexes for building pathkeys.)
+ * Unordered indexes will always have an indexscandir of ForwardScanDirection.
  *
  * 'indextotalcost' and 'indexselectivity' are saved in the IndexPath so that
  * we need not recompute them when considering using the same index in a