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, 1 insertions, 2 deletions

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 8ed55a3d0e2..6aa34412def 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -452,7 +452,7 @@ create_index_path(PlannerInfo *root,
 		indexscandir = NoMovementScanDirection;
 	}
 
-	pathnode->path.pathtype = T_IndexScan;
+	pathnode->path.pathtype = indexonly ? T_IndexOnlyScan : T_IndexScan;
 	pathnode->path.parent = rel;
 	pathnode->path.pathkeys = pathkeys;
 
@@ -470,7 +470,6 @@ create_index_path(PlannerInfo *root,
 
 	pathnode->isjoininner = (outer_rel != NULL);
 	pathnode->indexscandir = indexscandir;
-	pathnode->indexonly = indexonly;
 
 	if (outer_rel != NULL)
 	{