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, 1 deletions

diff --git a/src/backend/executor/execScan.c b/src/backend/executor/execScan.c
index d4ed2358564..42acc102c63 100644
--- a/src/backend/executor/execScan.c
+++ b/src/backend/executor/execScan.c
@@ -246,10 +246,17 @@ void
 ExecAssignScanProjectionInfo(ScanState *node)
 {
 	Scan	   *scan = (Scan *) node->ps.plan;
+	Index		varno;
+
+	/* Vars in an index-only scan's tlist should be INDEX_VAR */
+	if (IsA(scan, IndexOnlyScan))
+		varno = INDEX_VAR;
+	else
+		varno = scan->scanrelid;
 
 	if (tlist_matches_tupdesc(&node->ps,
 							  scan->plan.targetlist,
-							  scan->scanrelid,
+							  varno,
 							  node->ss_ScanTupleSlot->tts_tupleDescriptor))
 		node->ps.ps_ProjInfo = NULL;
 	else