Rearrange the implementation of index-only scans.

This commit changes index-only scans so that data is read directly from the
index tuple without first generating a faux heap tuple.  The only immediate
benefit is that indexes on system columns (such as OID) can be used in
index-only scans, but this is necessary infrastructure if we are ever to
support index-only scans on expression indexes.  The executor is now ready
for that, though the planner still needs substantial work to recognize
the possibility.

To do this, Vars in index-only plan nodes have to refer to index columns
not heap columns.  I introduced a new special varno, INDEX_VAR, to mark
such Vars to avoid confusion.  (In passing, this commit renames the two
existing special varnos to OUTER_VAR and INNER_VAR.)  This allows
ruleutils.c to handle them with logic similar to what we use for subplan
reference Vars.

Since index-only scans are now fundamentally different from regular
indexscans so far as their expression subtrees are concerned, I also chose
to change them to have their own plan node type (and hence, their own
executor source file).

3 files changed, 53 insertions, 6 deletions

diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index 5100642dd63..24ac5295f60 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -370,7 +370,31 @@ _copyIndexScan(IndexScan *from)
 	COPY_NODE_FIELD(indexorderby);
 	COPY_NODE_FIELD(indexorderbyorig);
 	COPY_SCALAR_FIELD(indexorderdir);
-	COPY_SCALAR_FIELD(indexonly);
+
+	return newnode;
+}
+
+/*
+ * _copyIndexOnlyScan
+ */
+static IndexOnlyScan *
+_copyIndexOnlyScan(IndexOnlyScan *from)
+{
+	IndexOnlyScan  *newnode = makeNode(IndexOnlyScan);
+
+	/*
+	 * copy node superclass fields
+	 */
+	CopyScanFields((Scan *) from, (Scan *) newnode);
+
+	/*
+	 * copy remainder of node
+	 */
+	COPY_SCALAR_FIELD(indexid);
+	COPY_NODE_FIELD(indexqual);
+	COPY_NODE_FIELD(indexorderby);
+	COPY_NODE_FIELD(indextlist);
+	COPY_SCALAR_FIELD(indexorderdir);
 
 	return newnode;
 }
@@ -3871,6 +3895,9 @@ copyObject(void *from)
 		case T_IndexScan:
 			retval = _copyIndexScan(from);
 			break;
+		case T_IndexOnlyScan:
+			retval = _copyIndexOnlyScan(from);
+			break;
 		case T_BitmapIndexScan:
 			retval = _copyBitmapIndexScan(from);
 			break;
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 9f564277747..eba3d6d5797 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -447,7 +447,20 @@ _outIndexScan(StringInfo str, IndexScan *node)
 	WRITE_NODE_FIELD(indexorderby);
 	WRITE_NODE_FIELD(indexorderbyorig);
 	WRITE_ENUM_FIELD(indexorderdir, ScanDirection);
-	WRITE_BOOL_FIELD(indexonly);
+}
+
+static void
+_outIndexOnlyScan(StringInfo str, IndexOnlyScan *node)
+{
+	WRITE_NODE_TYPE("INDEXONLYSCAN");
+
+	_outScanInfo(str, (Scan *) node);
+
+	WRITE_OID_FIELD(indexid);
+	WRITE_NODE_FIELD(indexqual);
+	WRITE_NODE_FIELD(indexorderby);
+	WRITE_NODE_FIELD(indextlist);
+	WRITE_ENUM_FIELD(indexorderdir, ScanDirection);
 }
 
 static void
@@ -1501,7 +1514,6 @@ _outIndexPath(StringInfo str, IndexPath *node)
 	WRITE_NODE_FIELD(indexorderbys);
 	WRITE_BOOL_FIELD(isjoininner);
 	WRITE_ENUM_FIELD(indexscandir, ScanDirection);
-	WRITE_BOOL_FIELD(indexonly);
 	WRITE_FLOAT_FIELD(indextotalcost, "%.2f");
 	WRITE_FLOAT_FIELD(indexselectivity, "%.4f");
 	WRITE_FLOAT_FIELD(rows, "%.0f");
@@ -1752,8 +1764,9 @@ _outIndexOptInfo(StringInfo str, IndexOptInfo *node)
 	WRITE_FLOAT_FIELD(tuples, "%.0f");
 	WRITE_INT_FIELD(ncolumns);
 	WRITE_OID_FIELD(relam);
-	WRITE_NODE_FIELD(indexprs);
+	/* indexprs is redundant since we print indextlist */
 	WRITE_NODE_FIELD(indpred);
+	WRITE_NODE_FIELD(indextlist);
 	WRITE_BOOL_FIELD(predOK);
 	WRITE_BOOL_FIELD(unique);
 	WRITE_BOOL_FIELD(hypothetical);
@@ -2707,6 +2720,9 @@ _outNode(StringInfo str, void *obj)
 			case T_IndexScan:
 				_outIndexScan(str, obj);
 				break;
+			case T_IndexOnlyScan:
+				_outIndexOnlyScan(str, obj);
+				break;
 			case T_BitmapIndexScan:
 				_outBitmapIndexScan(str, obj);
 				break;
diff --git a/src/backend/nodes/print.c b/src/backend/nodes/print.c
index 0b0cf384829..5fe4fd5520c 100644
--- a/src/backend/nodes/print.c
+++ b/src/backend/nodes/print.c
@@ -320,14 +320,18 @@ print_expr(Node *expr, List *rtable)
 
 		switch (var->varno)
 		{
-			case INNER:
+			case INNER_VAR:
 				relname = "INNER";
 				attname = "?";
 				break;
-			case OUTER:
+			case OUTER_VAR:
 				relname = "OUTER";
 				attname = "?";
 				break;
+			case INDEX_VAR:
+				relname = "INDEX";
+				attname = "?";
+				break;
 			default:
 				{
 					RangeTblEntry *rte;