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).

1 files changed, 8 insertions, 4 deletions

diff --git a/src/backend/executor/execUtils.c b/src/backend/executor/execUtils.c
index 4dbf10b8da9..65591e2445d 100644
--- a/src/backend/executor/execUtils.c
+++ b/src/backend/executor/execUtils.c
@@ -566,20 +566,22 @@ ExecBuildProjectionInfo(List *targetList,
 
 			switch (variable->varno)
 			{
-				case INNER:
+				case INNER_VAR:
 					varSlotOffsets[numSimpleVars] = offsetof(ExprContext,
 															 ecxt_innertuple);
 					if (projInfo->pi_lastInnerVar < attnum)
 						projInfo->pi_lastInnerVar = attnum;
 					break;
 
-				case OUTER:
+				case OUTER_VAR:
 					varSlotOffsets[numSimpleVars] = offsetof(ExprContext,
 															 ecxt_outertuple);
 					if (projInfo->pi_lastOuterVar < attnum)
 						projInfo->pi_lastOuterVar = attnum;
 					break;
 
+				/* INDEX_VAR is handled by default case */
+
 				default:
 					varSlotOffsets[numSimpleVars] = offsetof(ExprContext,
 															 ecxt_scantuple);
@@ -628,16 +630,18 @@ get_last_attnums(Node *node, ProjectionInfo *projInfo)
 
 		switch (variable->varno)
 		{
-			case INNER:
+			case INNER_VAR:
 				if (projInfo->pi_lastInnerVar < attnum)
 					projInfo->pi_lastInnerVar = attnum;
 				break;
 
-			case OUTER:
+			case OUTER_VAR:
 				if (projInfo->pi_lastOuterVar < attnum)
 					projInfo->pi_lastOuterVar = attnum;
 				break;
 
+			/* INDEX_VAR is handled by default case */
+
 			default:
 				if (projInfo->pi_lastScanVar < attnum)
 					projInfo->pi_lastScanVar = attnum;